Since pinions can get pricey, you may want to start with a rollout ratio of 1. Some people use that as a starting point. It tends to be quite conservative (low speeds but generally a very cool setup), so get that pinion and then a couple larger ones.
Use this
calc tool and enter all your data. Enter any reasonable pinion to get the results. In the results section there will be a little gear icon; when clicked it will bring up a table of the speeds achieved with various pinion sizes. The bold one is the one needed for 1:1 rollout ratio.