Yes, one size fits all. It's a flat sheet with a hole in it where the needle goes through when you are freemotion quilting.
Yes, you can move it from one machine to another. I usually tape mine down the to table/machine bed when I'm using it and just pull the tape up when I want to move it.
Yes, our LQS carries them, so be sure to check yours before ordering it.
And, yes, I really like it for freemotion quilting!