Generally,you want to go with a front/rear ratio of 2:1 or greater, meaning if the sprocket out back is a 20t, you'll want at least a 40t chainring. This is true for 3 speeds, the majority of which have the direct ratio on the 2nd gear. Some IGHs have the direct ratio as the lowest (first) gear, and these can run much smaller ratios. (The flipside is, you basically have to run a smaller direct ratio if the direct is the lowest gearing offered.)

Hope this helps. If yours is a raleigh-built hercules, be aware that none of the threads for the bb shell or fork will be the standards you may be used to. Also, the fork might be spaced oddly, for a very narrow (91mm) hub with a scrawny (5/16") axle.

Good luck with the build!