Smallest diameter round you will split?

Think you've got it figured out. Nothing less than 6" for me. If left round under 6" I try to get the bark off of it but if it doesn't come off readily, no worries.

I would split anything over 4" personally since I'm behind on my wood. In your case perhaps 5-6" is a good place for you. Additionally I'd make sure there's at least a cut through both layers of bark the entire length of the round. Birch is notorious for rotting if the bark is not opened since the bark is not water permeable. Most of my trees are soft woods with very thin bark and by the time I've lilbed the tree no rounds are completely covered in bark anymore, even at the top.

I was splitting anything 6" or above. But really everything dried out even if not split. In Pacific NW all we have is Pine. Quick Drying in the Sun and No Rain (Summer/Fall).

I think it has a lot to do with what stove you're running, your geographic location, how long the wood will be allowed to season and what kind of timber it is that you're considering splitting. Here in the rather wet northeast, I normally don't split anything about 4" and under for my stove if the rounds will have at least one season to dry. But again, that depends on the timber. I always split oak, even 3" rounds. I took down a bunch of green birch this winter that were bent over my road (due to ice and snow build up). I cut them into rounds back in January but just split them this week. Everything was split down to the 3" rounds because they were green when cut and I will be using them in around seven months. Smaller splits don't last long obviously however they work great for starting a cold stove.

Bottom line is what works best for you and if you want to put the work into it. Geographic location + type of stove + how long the wood will season + type of timber will give you an answer that works for you.

I split everything down to maybe 3”. Dries better, lights better, burns better. I have a fast splitter, so it doesn’t take me but an extra few seconds to split and stack, vs just stack it.

I like to keep a miss match of sizes. I have some ten inch diameter rounds of locust that I keep big for nice long over night or long days when we are at work. I have a lot split at 3 to 5 inch as well everyone's situation will differ a little.

anything 4" and under i give to the neighbor.

I only split if it won’t fit in the stove