Big is better - as large as you can conveniently fit. A small hot fire burning fast in a big stove is much more efficient and heats up much faster than the opposite. Though you have to feed it more often - it's a trade off, but you can fill it more and burn slow if you need to.
Flue - best to have as much as possible exposed with no insulation so that it also gives out heat.
Ours is a
Dowling Sumo, which also burns all sawdust and chippings very efficiently. The pyramid like shape means easy top filling and tight packed with saw dust it burns from the top. It takes 3 or 4 coal hods of chippings/dust. You have to let it burn down before you refill or you can get blow back, but no prob with larger pieces of wood. Not cheap though but worth it we think - fabricated steel is totally superior and maintenance free compared to cast iron and fire bricks.