I agree with some others who have said about 12" diameter to about 16". Closer to 12". I can cut it in rounds and throw it right on the truck. Much larger than 16" and I'll split it in the woods. My preference is to cut it, get it home, and then split a little bit at a time at my leisure.
Another vote for 12-16" wood. The size pieces that you split into 4-6 hunks seems to add the fastest, when all is said and done.
I find that whatever size gets' dropped off at my house for free is the most productive. That almost never happens though, I normally have to go cut it and haul it first. But I like the 6" - 16" range.
Plus it's nice when they split somewhat straight, instead of a lot of the stuff I end up with. But hey, it all burns.
