for the sake of brevity: hollow needs to be larger diameter for the same rate (usually), arm length and end link length both play a role in effective rate and "feel" of the bar (i know, very technical!)
for most of what people are doing here everybody seems to like the hellwig 1.25 hollow bars.
so without getting into an entire discussion about: material, wall thickness, motion event and installation ratio... depending on your t-bars, wheels/tires and shocks. something between 15/16" and 1.25" solid or hollow should be fine for your application.
