Welcome another Kitewingnut!
I don't use a harness with mine, but I'm on hard-packed surfaces ...
Medium sounds like it would be very close to right but I know how you feel - want to be 100% sure!
... try a store like kite-line.com, they looked after me here and I'll be buying my next harness from them!
FYI adding your location to your info will help with giving advice about brick'n'mortar stores too