Ever wonder how tall they are? Well, I answer that question... With SCIENCE!Note: "px" means Pixels!
We start with a scene from "Heart's Warming Eve", where Twilight says: "I spy... An 8-foot Candy Cane!"
She might be guesstimating, based off of the fillies and ponies nearby, or perhaps her magical telekinetic ability gives her the ability to "feel" an object and know it very well from a distance (Raven from Teen Titans (also voiced by Tara Strong, like Twilight) said when she moves objects, she extends her soul to them and becomes part of them)
Either way, it's by far our best shot, since apples can range in size greatly, but "8 feet" is pretty concrete, and knowing Twilight, she's probably not too far off.
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"My Little Pony: Friendship is Magic" is property of Hasbro.
Few things I'd like to note...
*Average person would be slightly below eye-level with Celestia. The ponies just might get the impression that humans are like deities because the only being they're used to looking up to and talking to like that is Celestia and Luna
*Their EYE LEVEL is at the "awkward place" on humans, not their HEIGHT. Their height, surprisingly enough, is rather tall: it comes up to about the chest. So they're quiet a bit bigger than I would've thought, or at least quiet a bit bigger than what I've seen on the internet.
*You'd be big, but not too awkward going through doorways and such, like with Celestia in the series.
*When they stand up, they're at the perfect height to pat them on the head
Though you may pat Rainbow Dash's head only at your own risk. She might not take well to being patronized
(She'd probably hover when you're around so she doesn't have to look up to you
(and especially after you pat her on the head))