TSA doesn't care what size bag you haul through, but common sense does apply. I try to carry on if I can because there is a time savings on the landing side. At the gate, if it is too big, they will stop you and may charge you. If it is of "proper size" or close, you might get on the plane with it or have to gate check. I was in Detroit a few weeks ago and the gate agent made me "try it in the sizer", of course it was too big without getting physical (I am talking about my suitcase Wolverine, not my privates), so she made me check it instead of carry on.