From my experience a usable reduction ratio (at least to start with) is 6 to 1. This applies to both electric and ICE powered bicycles, mini bikes and go karts. 40 years ago, a single speed mini bike would have a 10 or 12 tooth clutch sprocket and a 60 or 72 tooth rear sprocket. For some reason, this works out very well with bicycles. If you go friction drive, you want a 1" Ø roller for average conditions, smaller for hills and no more than 1-1/4" Ø for high speed use. Just my $.02 worth...

Cris