About 125mm or 5 inches diameter is perfect. Because most (about 80%) of the ‘fuel’ in wood has to be vapourised (turned into a gas) before burning, the surface area to volume ratio of the log is important.

The gas has to vapourise from the log surface, so if the log is large the surface area to volume ratio is lower, the gas doesn’t vapourise fast enough and the log will tend to ‘slumber’. Conversely, a smaller log has a greater surface area to volume ratio so gasses come faster. This is why kindling burns furiously.

What’s that got to do with log size? 

Big logs: Because most of the ‘fuel’ needs to heat up and ‘vapourise’ off the surface, log size makes a big difference. A large log has a small surface area relative to its volume, so the flammable gasses are given off slowly. As a result, the burning temperature falls, less of the gasses burn which causes inefficient burning and increased emissions. That’s no good for ‘Advanced Woodburning’.  

Small logs / kindling: Small pieces of wood have a large surface area relative to their volume, so the gasses vapourise quickly. This is why small kindling burns furiously. If you use too much kindling or too much wood with a large surface area (e.g. pallets), your stove won’t be able to deliver air fast enough to burn all the gasses. If this happens, the fire box will be full of flames, the unburned fuel gasses are wasted to the chimney and there will be lots of smoke out the top. So, people who are ‘Advanced’ woodburners only use kindling to start a fire or to re-light a dying one.