I believe all the friendship students are US high-school age, putting them at 15-16. Sandbar's body and limbs have similar proportions to an adult mare body, although it is a bit more squarish. He's definitely smaller than an adult stallion. With that in mind, I wanted a 15-16 filly body, so I took the adult mare proportions and made the limbs a little bit more slender, partway between adult mare and teenage AJ. The belly is also a little less rounded than an adult's.
Finally, I kinda like the ear style that Fleecs is using, which seems to be a hybrid of show and movie styles that IMO looks better than both. It captures the 3d aspect of the movie ears while remaining more show-faithful.