I have several piles of small trees and branches cut 6 momths ago or less.
Can I burn these this winter?
Also, I know it seems obvious what to do -- cut the leaves off, cut to stove length, stack it off ground under cover -- but I'm wondering what's the minimum I can do and still have it usable next winter. IE, will a brushpile have stove-worthy wood next winter if I do nothing to it at all? I need to prioritize other tasks right now as much as possible.
Assuming it gets at least some sun and is mostly loose and off the ground, it should be adequately seasoned by next winter. It might be seasoned enough for this winter, at least the smaller stuff. It won't be as dry as you want it, though, unless it has been under cover yet with ample air circulation for some months.
I would say the minimum you need to do now, or by next spring, is to put a sheet of plywood or something over the pile, with spacers so it doesn't crush it all down to the ground, and allows full air circulation. It may be easier to cut it up some and haul it to a convenient spot that can be sheltered.
I think twigs smaller than about 1/2" are generally too small for RMH fuel (except as kindling), as they will burn all to coals and collapse before they have time to finish burning in the airstream. They will build up a bed of coals that may not allow the bottom coals to get oxygen. Larger sticks will hold their shape longer and burn more completely.