To be a little more clear. The 30A is needed when running a heated bed for 3D printing, and works great for a diode based laser power supply. The machine runs on less than 3A if all the steppers are moving at the same time, even less on normal use.
Making tubes more rigid isn’t really a good idea, if you are worried you can get slightly thicker walled Stainless steel tubes. But gannicuss is right depending on what tool you use and what material you plan on cutting you can go a lot bigger, and the mid span braces really help here. I don’t think a full sheet is possible but you can get one axis really long if the other is shorter, a large square is not recommended. But bigger is less rigid, so build as small as absolutely possible.