You need to measure the pony really. A chunky will take up more rug than a fine build. I can never remember where to measure from, but I think it's from centre chest to point of buttock for most brands. Perhaps check with supplier or online to find out for sure?
Really depends on the pony and sometimes the make of rug,my little one is in a 5'0 a 4'9 will fit length ways but she is a bit porky so it is too tight width ways.
I reckon about a 5'0. 4'9 if she isn't too porky, but seeing as little ones generally carry a little too much weight then 5'0. Try on a second hand borrowed one..........
buy on a try if it doesnt fit return for next size. most tack shops are happy with this if yuu return rug in a clean condition. put a large cotton bed sheet on first!"