Here's a post I put up a while ago detailing my process: https://poal.co/s/DIY/627390 My source wood is just small branches that fall off trees in my wood lot. I have about 2 acres of Blue Gum trees that are used for fire wood. This provides me with as much wood for charcoal making as I would ever need. The key to making it is it really needs to be dry. So, I fill the retort, then leave it for a while to dry out thoroughly before lighting it. If it's at all damp, it won't burn well (as the heat is sucked out to evaporate the moisture), and you'll end up with partial pyrolyzation. As for wood size, I generally use branches that are up to the thickness of my arm. I cut them to the full length of my inner drum, and they will pyrolyze fine. You can go smaller, as small as you want, I often pack in offcuts from woodworking to fill up the gaps. That depends on what you want as the final product though. If you're after lump charcoal for smithing or cooking, go with larger branches. I wouldn't go much bigger than ~3" diameter though, otherwise the center might not pyrolyze fully Here's a pic of a more recent batch that shows the size of the wood I'm using: https://pic8.co/sh/IhGJVP.jpg