So I wanted to add my design on here:
I wanted mine to be as wide as possible for stability, particularly because of the big full-wall slide. I plan to add eyelets to attach the turnbuckles while it's on the dolly to make sure it's as stable as possible.
I also designed it to be as short as possible while allowing me to walk under the cabover portion without ducking, and still having it clear most of the garage ceiling (the steel I-beam you can see is 12' at the bottom).
The casters are 1200# each and I used the same steel as the OP. As with all things down with TLAR (that looks about right) engineering, it's probably overbuilt by about a factor of 10, but what isn't overbuilt is me -- takes every ounce of strength I have to move it around, so hopefully my wife will be able to help out pushing it around (fortunately she's quite athletic).
BTW, there's a sheet of plywood screwed down to it - just took the pic before to show detail.