I'm about 5'6" and I sometimes use a full size SAM-R (essentially an M16A4). It weighs in at a realistic 12 pounds. It requires some getting used to.
First and foremost, it depends on your fitness and upper body strength: if you can't hold the gun up it's pointless and you might as well get something smaller and lighter. There's a well known incident/joke involving a former user/Wanker who bailed on his squad mid-game because his big bad SR-25 was more than his scrawny hide could handle carrying around.
Assuming you can hold it up alright, you have to learn how to maneuver it properly. A full size rifle isn't a carbine or SMG, especially in CQB/FIBUA conditions. If you're looking for something you can just run around with without paying attention it, then get something smaller.
