Rotary Cutters come in 4 main sizes, 60mm, 45mm, 28mm and 18mm. The size refers to the diameter of the blade. The main size is a 45mm rotary cutter this will be your main cutter and if you only have one then this is probably the one to get.
A 60mm cutter is for larger cutting jobs, the larger size means that they can cut up to 8 layers of fabric.
28mm and 18mm are smaller diameter blades for finer or detail cutting. For example cutting double blunt corners off Marti Michell templates can be done with a 28mm diameter rotary cutter.
