Some of the more experienced SPs I see always have a range of sizes available, which is a godsend as I was spending extra cash buying my own for a while.
I'm also girthy, but have found larger sizes often slip off, so I've actually started going for standard size now. But, as I said, my last few punts especially, I've taken my own just in case but the SP had a good selection of sizes.
If you're really concerned try getting a variety box of Skyn (the best IMHO) and seeing which fits you best....