Image imported from derpibooru 
Original Description:
 
HERE’S MY HEADCANNON!  
So I dunno I feel like this makes the most sense.
 
I think Earth Ponies would be usually larger in structure and size since they’re the strongest and they (usually/traditionally) tend the land.
 
Unicorns (Twilight included since she started out as one) I see as more “dainty” and elegant. Since they are magical beings, I feel like they or maybe their horn would have a sort of shimmer to it. (More on Twilight in a moment)
 
And Pegasi I think they’d have to be smaller, and slimmer to be more aerodynamic. (Especially Dashie) They’d be able to swiftly fly through the wind and their wings would be very strong, enough so that they (for the most part) can fly against strong winds and storms.
 
Okay now Alicorns… I think they’d be kind of a combination of all these things. Elegant, strong, swift. I also feel like they’d (as usual) be the largest in size  
IF they were born an Alicorn. Since Twilight was turned into one, she grew a couple of inches, but she won’t get to their size. Only true fully blooded Alicorns are that large size.
 
And pictured are of course the mane six and I will list them from tallest to shortest in case it’s not clear… (this goes for my human mane six as well, except for the unicorns. I have Rarity taller than SciTwi)  
AJ, Pinkie, Twilight, Rarity, Fluttershy, and Rainbow Dash