I asked Jennifer what the height of the average pegasi and the height of Star's friends and she answered back! Answer :-) :

Sun Herd pegasi average between 15 and 16 hands. Mountain Herd steeds are a bit smaller. Desert Herd steeds average 16 to 17 hands and they're very lean. Snow Herd steeds average 17-18 hands, and Jungle Herd steeds range the most, from 14 to 17 hands. Star reaches 19 hands as a two year old, but he's not built like a draft. He's super athletic and muscular with a huge wingspan. Morningleaf is about 15.2, but still growing. Bumblewind was 16.3. Hazelwind is 16.1. Echofrost is 14.3. Shadepebble is 13.3 (runt). Dewberry is 15 hands.

This is just some information if you guys wanna know :-)