I'm not sure I understand the problem. Do you currently have a rotary cutter and need to buy replacement blades for it? And, if so, is your question "how do you know what size of replacement blades to purchase": 28mm, 35mm, or 60mm?
I can measure the width of each of these blades so that you can compare the size to your current blade if that will help. Let me know if that would be useful.