You might get away with 20mm screws on X if using rotating nuts. The only reason for using bigger screws is if you cant tension them sufficiently to avoid too much sag over the length. Smaller screws also means less inertia in the (smaller) ballnuts, but this is not so critical. The tension you apply will depend on the amount of distortion in the frame ends you're prepared to tolerate and that will depend on the cross-section of the frame material. What do you plan to use for the frame?