My first suggestion is that you construct your roll bar to a specific standard unless you simply want one for its looks. The SCCA is the largest amateur road racing organization out there, and most other clubs reference their standards when it comes to safety. Alternatively, NHRA dominates drag racing and their specs are completely different.
Again, if safety isn't your primary objective, anything goes.
Most roll cages are constructed from 1.5" x .095" DOM. Heavier cars require 1.75", even heavier (i.e. Mustangs & Firebirds) require 1.75" x .120" DOM. Roll bars are slightly different in requirements depending if you run Autocross (aka Solo 2) or Time Trials (aka Solo 1). It is also different depending on your modifications, speed potential and class.
Height is also a critical consideration, because the SCCA wants the bar 2" above your helmet. At 5'10" tall, my Solo 1 bar is 3" ABOVE the JH top! I have to bolt it in and out every time I race, but that is another story...
You can see the difference in a Solo 1 bar, and a 'decorative' bar in the two examples below.