Hi Laura,
When the Fluidity bar is folded up, the frame is 43 inches long and 33 inches high. If you leave it fully assembled and fold it up, the widest part of the frame measures slightly over 5 inches and the (removable) pad adds an additional 2-1/2 to 3 inches in width because it lays against the lower bar and is pushed out to one side. You can do more disassembling and get it down to 4 inches in width if necessary. If your bed has that kind of clearance, then it will fit. A lot of beds, though, have some support in the center so it isn't all open space under there. You might want to check that out. The bar weighs 55 pounds, but it is on wheels so once you get the thing out from under the bed and upright, you can just roll it wherever you want. It would be a hassle, but doable.
If it's free, I'd say go for it and then worry about where you will put it. It really doesn't take up too much space just laying against the wall, or maybe you could slide it into a closet.
Lucky you!
MissL