Little Ponies are little · 6:33am Feb 4th, 2014
One of the gems we pulled from the Pinkie Pride episode was actually pony heights. Cheese Sandwich appears in a lineup, with a scale behind him. From this, we can get his height, and compare him to other characters to get their heights.
There is one major, glaring flaw with these calculations: They measure from the top of a pony's head to the ground. This is understandable, as it's what you do when measuring a human's height, and we're humans (most of us...); unfortunately, that technique doesn't work for quadrupeds, who can move their head up and down a considerable distance, and do so frequently (especially the two characters measured in this image!)
Instead, you need to measure from the animal's back to the ground. In the case of horses, the standard is to measure from their withers, a bony ridge between their shoulder blades, right at the base of the animal's neck. (In fact, this is the standard for measuring dogs, too, so we could use it to get Winona's height!) Unfortunately, MLP ponies aren't rendered with withers, so we have to estimate. The point on the back where the neck and back meet is good.
Using the above screenshots, this puts Cheese Sandwich at 2.9ft and Pinkie at 2.04ft. In horse measurement terms, that's 8.7 and 6.12 hands high, respectively. (1hh = 4in) To reference another character known for being tall, I checked Big Mac against Pinkie (using Pinkie's numbers here and a screenshot from Magic Duel) and came up with 8.88hh.
In the real world, a "pony" is a horse breed which measures less than 14.2hh when fully grown, and more than 8.5-9.5hh. (Less than that is a "miniature horse," but the range is because of differing classification by differing breed registries.) With some margin for error*, that puts the MLP characters on the low end of "pony" height.
Ergo, we have a cartoon full of little ponies! 
I wrote all this up as a comment on another user's blog post, and when I was done, I went back to try and find Celestia's height. Using an image from Return to Harmony and comparing her to Pinkie, she measured 12.84hh, still well within the range of "pony!"
Yes, I have posted some of this information before, talking about how in my headcanon the ponies sit around 10hh. I was about 4 inches too high in the case of Big Mac. Or maybe I was just measuring from the tops of their heads...?
* Sources of error:
1. Finding Cheese's withers in the first image is difficult, since he's facing the camera.
2. The withers are actually slightly higher than the rest of a horse's back, but MLP ponies don't have one.
3. Comparing the scale on the wall behind Cheese to Cheese's height pixel-by-pixel is inaccurate, since he is not flush against the wall. (This is also a problem when extending this pattern to others, as they need to be the same distance from the camera for it to work correctly.)
4. The Mane Six may-or-may-not be fully grown. There are certainly adult characters taller than them.
5. The pony-vs-miniature-horse distinction is based on what's normal for the breed, not an individual specimen. Pinkie's measurement above may be under the minimum for "pony," but she might also just be short.
Wow, very neat deductive reasoning.
I love constructive overanalysis.
Though I don't think Pinkie's short. She does use the default body model, after all.
Also, given the amount of reality warping, I'm pretty sure the Goof-Off was an example of raksha shaping combat.
How do you know the scale depicts feet? There is no unit of measurement shown, and ponies might use a different measurement entirely. Just a thought.
You're also assuming that a race of magical equines uses the same units of measurements that we do. For all we know those are Metric units or something even more exotic.
1795349
1795650
Several reasons.
This is a children's cartoon, which skips a lot of world-building. There's little reason to assume the writers came up with an entirely new form of measurement for a one-use background picture. There are plenty of real-world things that make it into the show which make little sense for ponies to invent. (Why in the world would a pony invent a teacup handle, when nopony would be able to use it? Along similar lines, Cheese sang about "Hawaiian shirts." Perhaps he just mispronounced "Haywaiian?"
)
I use feet instead of meters primarily because, as a citizen of the US, that's the system of measurement I'm used to. While the show is produced in Canada, Canadians use a mix of imperial and metric measurements, and many of the staff members of the MLP team are from the US (who would be used to imperial). Additionally, it is extremely simple to convert inches/feet into hands high (1hh is exactly equal to 4in), which is the worldwide standard of measurement for horses.
Meters might be the next most logical choice for the scale after feet, which would put Pinkie Pie at a whopping 20.07hh -- just 0.61hh short of Big Jake, the tallest horse in the world. (If the scale were in inches or centimeters, even Big Mac would be under 1hh!)
So, here's the list:
1. Scale is in feet, Pinkie is just shy of the minimum "pony" height
2. Scale is in meters, Pinkie is just shy of the tallest horse in the world
3. Scale is in decimeters, Pinkie is ~1/4 the height of the smallest pony
4. Scale is in inches or centimeters, Pinkie is under 1hh
5. Scale is in some more exotic unit or a made-up unit, the scale is useless to viewers, not to mention unlikely given the low-quantity of world-building in the show.