I believe Euro size is 50mm
I have no idea what the US/Canada size is though.
Your forum profile is blank, so not exactly easy to see which country you are in
Original English size was 50.8mm ( 2" )
Even though we English were pleased to have bigger balls then the Germans, this traditionally caused numerous compatibility problems as you might imagine, until one manufacturer came up with a mechanical design that allowed the larger socket to be properly secured on the smaller ball.
In UK since around 1990, the 50mm ball has been the 'de facto' standard, so not really an issue now.