Split size- If the wood is not dry enough, it will not burn well, probably why you find the smaller pieces burn better. The best burn with marginal wood is to load "north/south" (ends pointing to the door) This gives the hottest burn. Loading "east/west" (ends to the sides of box) gives the longest burn, but not good if the wood is not perfectly seasoned.
This link explains it very well and also shows a good method of firestarting: you will have to copy and paste in your browser-
http://www.ec.gc.ca/cleanair-airpur/videosWoodstove_mgt-Eng.wmv
IF you have enough wood stacked up for a couple of years, this won't be a problem when you get into dry wood next year. Trick here is keeping a rotating supply on hand so you are not caught short and forced to make marginal wood burnable.
Keep in mind I burn softwood, and live in a very dry climate.
I like to use the largest pieces I can fit in the box. Problem here is, buying wood offers less choice of size than splitting it myself. Most wood dealers know splitting wood smaller makes a "fluffier" stack (more air space), some of the wood offered down here is not much more than kindling. Anything under 6 inches I prefer be left unsplit, small pieces seem to accumilate on there own.
Yes, larger pieces require more time to season, but for long burns, nothing serves better. If you are splitting your own wood, aim for larger splits, the smaller stuff will be a by-product. You will discover when going to the stack for a reload that you want the bigger stuff and the smaller will not be what you want, most of the time.
If the coal buildup won't allow big pieces to be loaded, it is time to rake them to the front (more air here, if left in the back of the stove they seem to last forever) and set thermostat to a higher setting, they will burn down in a couple of hours, allowing a fuller reload.
The grey ash will pack down over time and should be shoveled out as needed (I can go about 2 weeks). When it is soft and fluffy, quite a bit of it will suck out the stack with a good draft. Hardwood ash may be different, I don't have much experience with that. Do not remove all of the ash when you clean out, 1-2 inches in the bottom is a good thing.
The face of the combustor may appear clean, but I was surprised when I pulled out my combustor to replace it. The interior of the combs had deposits of very fine, but firm, white ash in them. I could not see this when it was in the stove, and almost missed it when I took it out. The combs were not clogged, but had a dense coating on the bottom of each cavity. I will soak this later to clean it and save for a spare, it is too good to chuck it. Some BKs have combustor combs that are verticle, and do not have the buildup, my 1990 vintage princess has the rectangular, horizontal combs, and that is apparently why the ash was allowed to buildup. It was still working, but not very efficient.
The problem is pulling out the combustor, the gasket will have to be replaced, and a good risk of damaging the cat due to the tight fit.
Wood burning is a lot like cooking, it takes practice to get a good result. When you get it right, it is good!