Sometimes these plastic animals seem to dwarf their "human" counterparts. Working around horses daily, I can tell you this...
The average saddle horse measures 15 - 16 hands at the withers (the withers are above the shoulders). Mules and ponies will be slightly less, say 12 - 14 hands. Heavy (draft) horses will be 17 - 18.5 hands. The current world record is 20 hands I think. 1 hand equals 4 inches.
There's your equation. A real pony measuring 12 hands, times 4 inches, will give you 48 inches. Simply divide by your chosen scale (1:20 in my case). Thus, a scale pony would be ~2.4" at the withers.

