Chainsaws come in a variety of sizes, and 18 inches is just one of the many sizes available. However, the size of the chainsaw does not typically refer to the overall chainsaw. Instead, it refers to the size of the chain and bar.
So an 18-inch chainsaw has an 18-inch chain and bar. The latter are the two components essential to a chainsaw’s cutting abilities. Some chainsaws feature a 10 to 12-inch chain and bar.
On the other end of the spectrum, are 20 plus inch chainsaws. These are usually big and gas-powered. Smaller 10 to 18-inch saws are typically electric. The size of the chainsaw you need depends on the work.
For light tasks such as pruning and cutting small shrubs, a 10 to 16-inch chainsaw will be ideal. This is especially if you have a small yard or garden to work with. For medium to big jobs such as cutting firewood and felling medium-sized trees a 16 to a 20-inch chainsaw is best.
Anything above 20 inches is suited for professionals such as lumberjacks. Typically the larger the chainsaw the more power required to run it. Therefore, 24-inch chainsaws require more power than a 12-inch chainsaw.
As a general rule of thumb, the best chainsaw for you is the one whose length is 23 inches longer than the diameter tree trunk or limb you intend to cut. As such if you want to cut a 10-inch wide tree a 12-inch saw will be ideal.
However, if you normally cut 16 inch wide logs or limbs, a larger chain size will be needed. It is possible to cut a thicker log with a smaller chain but it will take you a long time. With the appropriate chain size, it will only take you a single pass.