The pieces of coal used in mechanical stokers needed to be smaller than the pieces of coal used for hand firing. As a result, most coal docks had two chutes to each track. Stoker coal was about 1"-2" pieces, while hand fired coal was larger, from about 4" to 6".
Coal stoker augers (feed screws) would often jam on a chunk of coal that would not feed through the tube. The fireman would reverse the stoker engine, which would usually free the jam. Sometimes the chunk that jammed the auger would refuse to break, and the fireman would have to run the stoker engine in reverse until the large chunk was pushed to the rear of the coal bunker. Hopefully it would stay there until the bunker was emptied. Having stoker coal crushed to a smaller size than hand-fire coal was supposed to prevent this, but large chunks slipping through was not uncommon.
The largest coal-burning locomotives before the invention of the power stoker were around the 2-8-2 and 4-8-2 size such as Southern 4501. These would require two firemen when working hard.