Theres' two places that can help you (
here) RC drain calculator. You enter the gph and it gives the weir length with about a 1/4" over the weir lip. Other place is on RDO (
download) for their Overflow calculator spreadsheet. It's for a slotted weir but if using a straight weir put number of opening as 1. Can easily play with the variables to match the height you want against flow and length.
It all seems to go back to Bazins formula
Q = 0.66 x cB x (2g)0.66 x H1.5 http://www.aquatext.com/calcs/weir%20flow.htm and crank it out