What do you think? here is mine:
AppleJack:6'2" and muscular(due to Applebucking)
Twilight Sparkle:5'8" and average with a bit of baby fat(due to mostly lack of exercise in the entire series except for a few occasions)
Rarity:5'7" and thin but not too thin(because she has to keep her figure)
Rainbow Dash:5'6" and thin/muscular(due to being strong and fast)
Fluttershy:5'5" and average(I just see her that way)
Pinkie Pie:6'1"(Because I think earth ponies would be the tallest humanized due to genetics) and chubby(due to her eating sweets and the fact she can break the laws of physics so it would make sense that she would be to me at least)

Yeah for me it'd be more or less similar. Though sometimes I think Fluttershy's figure might be thin like Rarity. As we've seen, she's got a model's figure as a pony.

My headcanon is that Applejack would be the tallest and definitely muscular.

Next would be Fluttershy, since she does apparently have a model figure. So tall and thin would be her body type. Along with the fanon that she has huge gazongas.

Next would be Rarity, an inch or two shorter than Flutters.

Then there is Pinkie who I see as having a an average height for a woman her age. She might be chubby, but she is also pretty active. I see her as one of those people who could eat as much as they want, but never gain a pound.

Then there would be Twilight, who I see as being kind of a stick. She stays inside all the time, but doesn't have unhealthy eating habits so I see her as kind of twiggy.

Lastly, there is Rainbow Dash who I see has being the shortest, but certainly with an athletic build. I don't know why, I just like the idea of her getting mad at constant short jokes lol.

tl;dr - Applejack>Fluttershy>Rarity>Pinkie Pie>Twilight>RD in height.

1991545 I always thought of twi as skinnier than average, cause of her forgetting to eat when she studied.

Twilight: 5'8", average body, modest B-cup
Applejack: 5'11", muscular, large C-cup
Rarity: 5'10", slender, large C-cup
Fluttershy: 5'10", willowy, D-cup
Rainbow Dash: 5'6", very athletic body, smallest breasts of the Mane Six
Pinkie Pie: 5'8", average body, happy B-cup

1991545
Applejack: 6'4, muscular, with dark skin and a large bust.
Twilight: 5'7, thin, with dark skin and and average bust.
Rarity: 5'11, an hourglass build, with creamy skin and an above-average bust.
Pinkie Pie: 5'6, a childish build, with tanned skin and a small bust.
Rainbow Dash: 5'4, a swimmer's build, with lightly tanned skin and a small bust.
Fluttershy: 5'9, a wispy build, with creamy skin and an average bust.

Though with Applejack being the exception, I can deal with the other's changing in accordance to the writer.

1992112 I always pictured Fluttershy to have the largest bust. Probably due to her shy and timid nature.

I can't really imagine girls being over 6 feet for the most part, it's very rare. I usually see them between 5'2" and 5'10", but heights change sometimes in my headcanons by story. Like I can imagine fluttershy being both short or tall and lanky, and her tits could be modest or large. Pinkie is kinda the same though I also see her as chubby. I always consider RD the smallest though, speed and athletics reasons. But considering how close they are in height in EG and similar in MLP, though that's probably just for ease of animation, I imagine they're all pretty close in height. I know lots of my friends in school were pretty much the same stature and body type, save one or two. But headcanons are all different, so whatever works for each of us... works for each of us.

this is what i think

http://askhumanappledash.tumblr.com/image/41838967095

except rarity is white.

1991545
Tan skin from working on the farm, medium height, medium to small chest, decently muscular
Latina, shortest of the group (in The Cutie Mark Chronicles she's extremely tiny), medium to small chest (but she wears a sports bra 24/7, so she looks flat chested), petite build, strong
Very dark skin (IDK I just picture her as African American / Jamaican / black), second shortest of the group, medium chest, kinda pudgy but not super overweight (she is really hyper so she burns most of the calories off pretty quickly)
I think of her as Indian, medium height, medium chest, pretty skinny (she doesn't work out too much but she probably skips a few meals per week to study and makes good choices, food-wise), medium to small chest
Very pale, second tallest, curvy, hourglass figure- medium to large chest
Second lightest skin (medium Caucasian), tallest of the group (about 5' 8"), very thin (model-like figure)
Hope this helps

Twilight: Average height, slim though not as much as Rarity. Average chest size, slightly dark skin in an exotic kind of way.
Rarity: Second tallest, about 5'8 or 5'9. Slim, but not stick-thin. Above average chest size, pale skinned but not in elegant, not sickly manner.
Applejack: Tallest, about 5'10, muscular, but not in a bulky or intimidating way. Above average chest size, tanned, naturally.
Pinkie: Second shortest, average build with constant snacking and calorie-burning hyperactivity canceling each other out. Regular skin tone, but maybe a bit chubby in the face. Average to slightly below average chest size
Rainbow: Shortest, about 5'4 or 5'5, has a sleek, athletic build, and is tanned. Smallest chest size
Fluttershy: Average height, with a model like build she is naturally embarrassed about and hides underneath thick sweaters and pants. Above average chest size, pale skinned.

1991545 Fluttershy is too damn small. Make her and rainbow Dash 6 to 5.8 feets tall.

In my opinion:
Twilight: 5'7" with an average weight (not too skinny from skipping meals but not to chubby from lack of exercise) and average breast size. I picture her face being rather cute and people are surprised to see her beauty on occasions.
Pinkie Pie: 5'8" and thick with slight curves and average breast size. I think she wouldn't be too chubby due to the amount of exercise she has. (Dancing, chasing Rainbow Dash, finding out how fast she can run from the swimming pond to Sweet Apple Acres, ect.)
Applejack: 5'10" Muscular, mostly in the legs from bucking apples. Tallest of the group, larger breasts. She appreciates being tallest and most muscular since she will inherit the farm and has her work cut out for her.
Rainbow Dash: 5'5 shortest (since a picture her a speedy little shorty ) and small breast. the combination of small stature and breasts makes her a target for teasing, which only motivates her to prove herself.
Rarity: 5'8" and skinny with lovely curves and average breasts. She works hard to keep her model figure and clear complexion since she wants to use her own figure as a model for her clothes.
Fluttershy: 5'8" with curves and the largest breast size. Yes, I've fallen victim to the fanon of her boob size, but I sit it fitting to her character. Being vegetarian and naturally inheriting her figure, Fluttershy is simply stunning without trying. (Although I think as a child she had braces or was too skinny, therefore teased)
I didn't mention skin tones because I didn't know if you were going to use the EG style of colorful skin. Which isn't necessarily bad, but try to stay true to the brightness/darkness of the pony's colors.

1991545
- Tallest, slim build, athletic type of body, no curves, simple, thin bod. Uses a chest binder to hide her huge *ahem* ta-tas? Slightly tan skin, from always being out in the sun and exercising.

- Tall, but not that much, a few inches above your average height. Perfect mix of curvy and muscular. Has a decent size chest, noticeable but not "WHOA" and her skin is heavily tanned from always working on the farm.

- Only a few centimeters above average height, lanky, skinny, somewhat figureless but has a bit of a body / chest. Nothing too special, but dorky average in a cute way. Pale skin from always locking herself in the library.

- Medium height, curvy waist, model-type body, obviously DD. Has a bit of meat on her bones, but isn't too chubby; sort of the cute amount that's also sexy. Olive skin from being out in the sun a little too much, although too much can cause crazy burn.

- A little less than medium height. Kinda chubby, but keeps it alright. Has the sort of adorable fat that's noticeable but not in a bad way. Small chest. Average Caucasian skin.

- The shortest of the group (but she always wears heels to make up for it). Curvy and almost like a model, but slightly lacking in some places. Average chest. Albino skin that burns very easily.

How about this?

- Applejack: 5'10. DD-cup. Muscular with visible six-pack. Hourglass shape. Similar colour scheme to EG!Applejack but skin is more of an orange tan than pure orange.
- Fluttershy: 5'8. DD-cup. Thin. Inverted triangle shape. Peach coloured skin with blonde hair the colour of Pony!Fluttershy's coat.
- Pinkie Pie: 5'5. C-cup. Chubby. Pear-shaped. Dark skin with red hair.
- Rarity: 5'7. C-cup. Slender. Hourglass shape. Pale skin and black hair.
- Rainbow Dash: 5'1. A-cup. Slender but stronger than she looks. Long legs and faint six-pack. Peach coloured skin and blue eyes the colour of Pony!Rainbow's coat. Black hair dyed rainbow.
Twilight Sparkle: 5'6. C-cup. Average weight. Triangle shape (wide hips). Average colored skin. Indigo eyes and black hair with magenta streak.