Depends on what you're cutting of course, but for general 2.5D machining I use the 6mm single flute cutters. I quite regularly use the same style cutters from 3mm to 8mm. The tool deflection is proportional to the length of the tool to the power 4, so a small increase in the length of the tool can lead to a much higher deflection and thus poorer finish, hence you should always buy the smallest ones you can use, so ideally keep a selection...