I typically load my stove twice a day no matter what the temps and vary the load with anticipated usage. My OWB is a Woodmaster which is just a round firebox with over fire air injection. My prior heating system, which was an add-on in the basement also had over fire fresh air, what I found with it (and its the same with the Woodmaster) is that constant loading of more wood than it uses results in a firebox full of coals that either have to be shoveled out and thus wasted or you have to spend a Saturday stirring and fooling with them to get them burned down. All in all is just easier for me to try to load what is going to be burned up more or less completely in 12 hours. Now if you are introducing the combustion air through a set of grates up through the fire then there is no (or at least less) worry with this. I do like the over fire air, its just a little more involved than just pitching the thing full of wood. Back to your question, with my boiler and my BTU needs I may put in two slit pieces twice a day in the fall and spring or I have filled the thing to the door when it was below zero and wind blowing. Usually I put in bigger pieces and fill the voids with smaller ones. The coals left over in the stove are raked forward, towards the incoming air and wood is thrown in the rear.
I have yet to experience the "rounds work better" phenomena that some speak of, I now work my wood as I did when I had two stoves to feed so it is good and dry and doesn't wrench my back throwing it in.
