As per above, a 5-point rollbar (most people make them 6-point with a side bar on the passenger side as well), you need 1.75" minimum. I've known a few people who started with that shape, then added the additional bars in 1-5/8" OD when they upgraded to a full cage.
One thing to be concerned with on a full cage (or a rollbar that may be later upgraded to a cage) - they are very strict on the 0.118" minimum wall thickness, and some nominal 0.120" wall tubing ends up less than 0.118" with tolerances. Because of that, most people opt for thicker nominal wall thicknesses.
Also be prepared for sticker shock when you get your cage certified - instead of the $75 mentioned in the link above, it is now $150 plus inspector fees (around $200 total depending on travel costs).
Regards,
Michael Pliska