How do I know what size hubcentric spacers to purchase?

After you have checked the minimum size that you can fit, you will need to measure to determine what thickness is suitable for your set up. This is personal preference and depends on what you're looking to achieve. To do this, you can measure the distance from where the wheel is now to where you want the wheel to sit in the arch.

If you are fitting studded wheel spacers, please see Measuring for Studded Wheel Spacers/Adapters