Denim and sisal I never use for anything
Size depends entirely on the speed of your buffer and what you're buffing. For the bulk of blade steel work you want a minimum of a 12" buff at 1750 rpm, and a minimum of a 6" buff at 3600. You also want something hard, and spiral sewn. 1/4" spacing at minimum, I prefer 1/8". Wider wheels are good too, and you can stack up as many as you want. One thing to keep in mind with hard wheels is they need frequent raking. My buff sees the rake every time I step to the buffer, and every 5-10 minutes of continuous buffing. It seems excessive, but keeping a fresh head of compound is both faster and makes for better results.
For handle materials (other than micarta which gets my big hard buff) I generally go back a size or two of buff. 8-10" on 1750 and 4-5" on 3600. As for the type, you can use either a fairly soft concentric sewn buff, or a loose one. Flannel buffs work too. Generally keep a few different buffs around for different handle materials, and keep them clean.
I always used ZAM compound for handles, but I tried a stick of gesswein lustre bar recently, and switched to it for all my handles last week. It's as big an improvement over Zam as Zam is over pink scratchless. By miles the best I've ever used.
I'll add that terminology on these is much like cutting tools. They're buffs and not buffing wheels, much like the things you make holes with are twist drills not drill bits. It's technically correct but virtually no one does it, so you may as well just ignore it
