How do you pick a grit size?

A quick and simple answer is to pick a maximum grain size that is about equal to the thickness of the coating being removed.  So if what you are trying to remove is 1.6 mm thick (.063 inch), you would choose a 12 grit.  A bit finer size is often used as well, because you get better coverage as the finer particles fil into grooves, cracks, voids in the surface that the larger particles will not fit into.

A more detailed answer involves selecting a grit size based on the surface profile you are attempting to achieve on your substrate.  A quick guide can be found here.

Keep in mind that the more rounded a particle is, the more contact area it has with your substrate, and this can speed up cleaning rates.  Glass bead is a good example of a cleaning abrasive, whereas the more angular and blocky aluminum oxide particle is useful for actual surface preparation.  (Where you want the abrasive to leave a surface profile for a future coating).