The big pieces do burn more slowly, but there's a downside to that. I drive by OWB's all the time that have huge stacks of unsplit rounds piled nearby, and in my experience I wonder how they can get away with that. I tried loading up the boiler with unsplit rounds once or twice and found that the fire never really got very hot, and the next day I would be left with a couple of charred chunks smoldering in the firebox and no bed of coals to speak of. Similarly, a lot of little pieces tends to make a huge pile of very hot coals but a short burn time.
So it's a mix. I load the OWB with a few split pieces, a few small pieces or rounds, some scraps, and a big chunk or two of something ugly, unless there's a good hot fire already and I can skip the little stuff. There really is a learning curve to this, and I'm learning... I split everything that will split easily, one or two shots with a maul and if it doesn't split, it gets cut to a manageable size with the chainsaw and goes on a nearby pallet for the "overnighter" pieces. I spent the first year with the OWB burning nothing but pine slabs and pallets and that made for some interesting ups-and-downs with the heat cycle.
So it's a mix. I load the OWB with a few split pieces, a few small pieces or rounds, some scraps, and a big chunk or two of something ugly, unless there's a good hot fire already and I can skip the little stuff. There really is a learning curve to this, and I'm learning... I split everything that will split easily, one or two shots with a maul and if it doesn't split, it gets cut to a manageable size with the chainsaw and goes on a nearby pallet for the "overnighter" pieces. I spent the first year with the OWB burning nothing but pine slabs and pallets and that made for some interesting ups-and-downs with the heat cycle.