I'm learning as I go (been running OWB for a year continuously and for a year before that off and on while building the house). I have found that it's best to split as much as you're willing to physically handle. The OWB will burn green wood fine, and I tend to use it to "preheat" and dry large pieces sometimes, especially large knotty stuff that's been sitting around a while. But for efficiency, and saving your back, smaller split pieces dry better, burn better, and are easier to handle. Sometimes I'm happy just tossing two or three big chunks in there, especially on a wet cold day where I don't wanna be out there feeding the beast. Other days, I'll go out, use the time to carefully load clean split pieces, take my time, enjoy the day etc. I split what's easy to split with a maul and leave the rest.