Sunny; race safe start in a medium (the smaller ones are a different design) I bought that size (fitted when we went to Olympia) when my daughter was 7. But it's the back length you need to be careful of (too long causes problems) they are better if fitted.
I think that they are one of the very few items which really need to be properly fitted as they can actually be really dangerous if they don't fit properly, eg cause neck or back injuries due to poor fit.
The different brands are quite different in fit. My dd has an airowear which they had to specially make for her as she is slim and tall, which the factory did at no extra cost. We are now switching to a racesafe, which will be made to measure at no extra cost. Never buy them with room to grow, it is dangerous and then your dc is better off without a bodyprotector altogether.
Cant help I'm afraid I have a similar problem - stables said to buy my 7 year old daughter (normal height/slim build) a racesafe body protector, which I will buy on-line, but they only seem to come in medium.
I waas hoping you could help. DS who is 6 needs a new body protector, and not having any tack shops remotely near us I'm having to order it online. I've done his measurements and according to the sizing guide he sits somewhere between an XS and S, should I be sizing slightly up, ie the S baring in mind he'll probably have a sweatshirt under his bp or should I go snug? The measurements are done in ballparks if that makes any sense?