If you made an airtight sphere out of ABS, in a perfect vacuum, with an edge thickness of 0.5mm, you'd have to have a radius of roughly 13 meters for it to float in mid-air.

Just letting you know.

Good to know. But I think 14.7psi would crush that 0.5mm like an eggshell.

FYI, I used to dream of a "1-man hot air balloon", that instead of a traditional basket, you'd have a harness you'd strap into, and dangle below it. Blew my mind everytime I calculated how big even a 1-man no frills balloon would have to be.

Rather than figuring out how many angels can dance on a pin head made from unobtanium, what about bringing the idea down to earth?

We could make RC dirigible or RC plane skeletons with a RepRap and then skin them with generic RC plane mylar or nylon, as hobbyists do.

This will do until we figure out how to weave carbon nanotubes and all that.