I have stolen everyone's idea

on here and went to Petsmart and bought some walnut shells and some corncob for my tumbler. The corncob seems to have have pretty good sized pieces. Everyone talks about it works better when it is fine ground, so do I need to grind it to a smaller size orwill it work good enough the size it is? Also what is a good polish to use that I can get locally? I don't want to have to place another order to Midway if I can get something locally. Also is it better to mix the mediait or tumble individually?