You're welcome! :) Apologies if I mention things you already know btw, I don't know what you know :p
If I get to pick I'd say square, but sometimes rectangles just make more sense. I'm using 32x32 squares for most sprites right now (both legs and upper body) and 64x64 for the firing torso sprites, just because the muzzle flash takes some extra space
It's really up to you though. I could share how I've set it up so far if you'd like to see how I'm setting things up?👀