What length chainsaw should I buy?

When deciding what bar length you need, remember that the diameter of log it can cut is roughly half the bar's length. So a chainsaw with a 12" guide bar will handle a 24" diameter log.

The most commonly purchased Guide Bar lengths for Petrol Chainsaws is between 16" to 20" and with Electric Chainsaws the most popular range between 8"–16".

Don’t purchase a chainsaw bar that's too short for the tree or branches you have to cut and remember not to land yourself with a bar too long.  A long guide bar with a smaller engine is not as useful, not least because balance is very important and balance helps adjust the weight. (See - How do I choose the right weight chainsaw?.)