Note: I'm by no means an expert on the subject and learn by testing out my own rigs.
I converted one pipe gun to a roller last year and built a wood roller gun as well, here are some of my findings after testing out for a couple months.(Bands loaded in a safe location, temporarily just for photo before being unloaded)
Small ID 14mm Primeline
Regular ID 14mm Primeline(minimal difference in felt power between the two, small id harder to load but easier than 16mm)
Shafts (all tricut):
Addiction Custom 7.5mm
Red Tide 5/16"
I converted an 80cm pathos and built an 80cm rear handle (note they were single stage, one pull to load).
A couple things I noticed with the thicker shafts:
-->Minimal to No shaft whip
--> Shooting reef fish under 5lbs, the fish usually spin on the shaft after being hit
--> I could shoot into holes and not worry about bending a shaft
--> Shots feel like they have more punch
--> Thicker fish like big sheephead I get full penetration (behind gill plate)
Using a 14mm band some things I noticed:
--> Hand picked 14mm (9/16") rubber that was super stretchy
--> Easy to load
--> Release seems progressive rather than explosiveTL;DR
I like 7.5mm-8mm for all around applications roller and non-roller and generally shoot 14mm (9/16") bands.