Getting out to games is a good way to check out what others are using. At teh same time you can get yourself age verified to open up the buy and sell section.
I'm a big guy as well and my first gun was an MP5. Just seemed to tuck into it very easily. I've since picked up a TM m14. Great gun for a bigger guy.
Most guys will let you hold their guns if you ask nice and are respectfull of them. That way you can try them out and see which one fits the best for you.
Nothing says loving, like a head shot!