I would talk to the longarmer before going any further. There's been a lot of discussion in the longarm groups that I belong to about T-shirt quilts. Some longarmers don't do them at all, while others specialize in them. Be sure that the person you go to has some experience with them. This person can also tell you about the type of batting that works best, how much bigger it must be, etc. I don't do longarming for others, but on my own quilts I want 3-4 inches extra batting and backing on each side so I can test my stitch length and tension off the quilt. (I add a piece of scrap top fabric to do this.) I need a little extra top and bottom, but not as much as on the sides. However - what you think of the sides may turn out to be the top and bottom on the longarm, depending on the size of the quilt and the size of the quilting frame.