For a massage, do most spas offer a choice between a sheet or a towel?
no, they don't. If you have a big preference for one, either let them know in advance or tell them as soon as you see the one you don't like.
I would like to try a body treatment but was wondering if men usually get these and are these clothing optional.
Men do get body treatments. Not sure by what you mean as "most" - most women don't get body treatments, as in a huge percentage of the population never sets foot inside a spa. Of the small percentage of the population that does go to spas, a lot are women, but certainly not all.