As to powder coating: you can do both if you wish....size before and after the coating. Since you're trying to increase diameter, I'd say coat and then size. Powder coating allows sizing of up to .005" without affecting the coating.
I use parchment paper as a non-stick base when powder coating, but a lot of people use non-stick aluminum foil. Either way, that "puddle" under the baked bullets is problematic as it promotes chipping the coating on the base of the bullet. If you tap each bullet to remove excess powder before placing it on the tray, you can eliminate the "puddle".
