How do I find out his ring size?

Me and my fiancé are engaged and have been for about 3 years! Wedding wise we want different things. I want small and discreet but he wants a fairly big one as he has lots of friends. I have major confidence issues so that's why I want a small, intimate one. Anyway, I was thinking of getting him a silicone wedding ring just so it's like we are married. I mean we practically are anyway. Not even sure if he'll wear it but they're cheap and look good, so if he didn't wear it then it wouldn't be a big thing. I have no idea what size to get though. I don't wanna ask as he'd know what I was up to.