How do I figure out his size?

I am trying to order my husband a robe online but not sure which size to get him. He is 5'10 and 160lbs. I am not sure if I should get him a large, medium, or extra large. I don't want him swimming in it but large enough so that it is comfortable and plushy. It is for Christmas so I don't want to just ask him.How do I figure out his size?
I'm that size and I'm a large. Medium is too small.How do I figure out his size?
Buy a large
Look at the size of his shirts, that should give you the answer. Most sizes are based on chest size so this is the best option.
i would say large he is about 10 lbs smaller then i am so i would go with the large the XL is gonna be too big
I would say MEDIUM!
I'm about the same size, and take a ';large'; in T-shirt sizes.
large...any excess will just wrap around him
size large they are made a little big
Medium should be O K.
Large. That's my size. I'm 5'10'' 160.