Welcome!! If you look in the “Builds, and Things Built” section, many of us have posted info on our builds and the sizes. The biggest I have seen is the 4’x7.5′. I guess it all depend on what you plan to do with it. If you give us a little more info on what you plan to cut, print, burn… it might help.
Structurally, filling the conduit with anything will not make it stronger. The strength is in the outside of the conduit. A tube is stronger than a solid bar of the same diameter. I don’t think the strength of the conduit is the limiting factor in many cases, and there are tradeoff’s.
Good luck on your build and let us know how your progress it doing.