Dec shot size?
As far as shot size goes, once there is snow on the ground, I go to heavier shot and a tighter choke with both 12 and 20. I usually start out fairly open, IC or LM and as the grass gets packed down, the birds get skittish and the snow impedes things, I usually go to a M or IM choke in my 20, I haven't hunted with my 12 late in the year as I just got it this year but would imagine the same scenario applies. As far as shot size goes, I usually start off with 6 or so but move to 4 or 5 lead. 4 steel isn't too big, problem with steel though is that it loses a lot of kinetic energy quickly and you may not get the penetration you're looking for on those long flushes that are more common later in the year. Really it comes down to the distance of the flushes more than anything else. If you have a bunch of steel shot sitting around, by all means use it up. I only use it when I have to though. I'd rather throw lead.