on the frames, they are made so that the space between the rollup bar and the front bar are designed for the throat size. so if you have a 24" throat, you need a much larger working field than if you have say a 9" throat. I THINK that's because someone decided that if only a sideways twin would fit within the throat of the machine, you shouldn't need more than 'x' inches on the rollup bar.
truthfully, if you want to COMFORTABLY do a queen in one piece on a machine, you need at least 15". if you want to do it on a home machine, you can do it in sections, sew the sections face-to-face, and slipstitch on the back. if that gives you time to save for the right machine for you, then do it.
remember, there's no such thing as temporary.