I have a Voyager 17 with Hinterberg frame, which is more like a midarm than a longarm. For setups where you move the machine, it is really the frame that takes up all the space. I'd have to go measure to be sure, but my frame is about 4 feet deep but you then need at least 3 additional feet at the front for your body. Most people also want 2 to 3 additional feet of space at the back to get around the frame for plugs, working on the machine, and to do pantographs from the back (3 feet). The width of the frame depends on the biggest size quilt you want to be able to mount. Mine is 10 feet wide, and I can fit a quilt up to 9 feet on it (you need to allow at least one foot on one side of the frame so you can put bobbins in the machine.
There are a couple of frames out that are smaller. I've seen questions about them posted recently on the QB. I think one is a Babylock. These are not the usual setup in that you need to layer the quilt as usual and then clamp just portions of the quilt to the frame while you move the machine to quilt.