depends what flow rate you're aiming for, and wat purpous you're using your sump for.
some people go with a coast to coats overflow which generally uses 3 1-2 inch overflows. coast to coast are genereally the best type to use as it skims the entire surface of the water so that the organics slick can all be drained.
a 1 inch gravity feed overflow can generally do 600gph and a 2 inch can do 2400 gph
it is best to have atleast two overflows just incase one clogs up with algaes or dead fish.
Pipe Sizing Charts and Flow Rates
heres a good arctical on flow rates in overflows