Aim for the largest that you can reliably pick the part with, allow for the worst case resolution on your tape feeder as well as the nozzle placement.

If physically aligning the part on the nozzle you may need to snatch it back when the tapper opens so a good flow rate helps.

OTOH, if you don't have an autochanger you may decide to compromise on a single tool that picks your entire selection of discretes.

Suck it and see