It all depends on what size front bar do you have now or are looking to get. Your rear shouldn't be more than about 65%-70% of the diameter of the front bar due to increased oversteer. VB&P have front and rear bar kits that match.
I'd recommend a 5/8th (15mm) bar at the most for most C3's, but then it all depends on how you like to drive!
You'd probably have a 1 inch bar on the front now?
I borrowed a 5/8th for my old '77 and I loved it! But I also had stiffer springs all round. I have a Greenwood suspension book and he says go soft springs and BIG sway bars.
Most bars are solid - never tried tubular.