Hi Budboy, if you took a look at some nut coal you could see that it would be impossible to get a piece of coal that large through the stoker mechanism and onto the burning grate. If an oversize piece of coal gets into a stoker mechanism, it usually jams the stoker and the fire goes out. They are designed for a specific size coal.
Rice and Buckwheat/rice mix are what the LL and other bed-type stokers use. Ohter types of stokers used on bigger boilers and furnaces may use larger coal. But they are not room heaters like the LL units.