I do all sizes. The way it works is that the quilt rolls onto a roller as you quilt- You quilt all the way across- and at the beginning, there is less on the roll, which is in the throat of the machine. So you have about 6 inches deep, across the whole quilt, to work in. You quilt that, and then roll that portion up and the next area comes up. You quilt that area, etc... But by the time you get to the end of the quilt, most of the quilt is rolled tightly on the roller, which is taking up more space in the throat of the machine.
Don't get me wrong- I love my machine, and being able to quilt on the frame. I also use that machine to make bags, piece, etc. If you have a machine that you love to piece with, I would probably suggest that you buy a long arm. If you think you are going to really like machine quilting and are going to use it alot.
Go to your local shops, or to a quilt show, and play with the set-ups, so you can see what we are talking about. This is the best way.