What size tube do i need for a roll bar?

I have got hold of a 12 ton tubella pipe bender with formers.

the pipe bender won't be much good for bending tube unfortunately. tried it before, and it just kinks the tube

tube and pipe are a bit different really. those things are more for thick walled gas pipes
roll bar tube is too thin

Hydraulic tubing used in large earthmoving equipment is thick walled and 0.5" bore. I recollect Racetech or possibly Racecar Engineering carrying an article by Graham Templeton on it with loads of interesting "what to use and where to get" stuff on it along with the usual calcs.

You should be able to access it on line