If you're looking to attach a chainsaw to a mill, the size of the saw will depend on the type and size of mill you have. For example, if you have a small portable mill, you may only need a small chainsaw. However, if you have a large stationary mill, you'll need a larger chainsaw that can handle the extra power and weight. In general, it's best to err on the side of caution and get a slightly bigger saw than you think you'll need - it's always easier to downsize than upgrade later on!