Well, I barelly heard about this pine species, and didn't know that other species, but most of the pines are really difficult to back bud, so people don't let them thick then chop, the approach then would be ramify, and use the branches to thicken it...
The Redbud should be easier to develop a trunk and chop back, or prune it and work the ramifications into sacrifice tips.
How big, is always on what size of Bonsai you want for the future, just have in mind that bigger the leaves, bigger the bonsai, so it can have proportion...
A really commom technique applied to Pines, is the sacrifice branch, which consist in let a branch grows as long as possible to thicken the base, keeping the low green mass... I've read that as long as you keep the lower needles, the pine should bud on the them... But well, there are many pine varieties out there...
But in general, you can ramify the lower part, and let the top grow freely.
This Bonsai Empire video may help you with the idea of the sacrifice branch...
About the pot size... The bigger it is, the faster it grows, and the bigger it grows, the thicker it gets... (Of course having some common sense, for there would be no reason to place a small seedling on a 10L bucket right away... xD) But as I learnt with Tropfrog, Pines don't like to be repoted very often... =]