What size conversion kit do I need?

The information you need to know will be written on the tyre of the bike you’re converting. It’s not the diameter of the rim alone, it’s a complicated measurement that is annoying to work out just once. The easiest, most straightforward way to know what size you need is to look at the info on the tyre. It should read 20, 24, 26, 28 or 700C somewhere on the sidewall.