Q = (K / N ) * A^(2/3) * S^(1/2).

Where Q=flow (CFS)
K=constant, 1.486
N=changes based on surface; grass 0.03, concrete 0.013
A=cross section area of flow (SF)
P=wetted perimeter, length flow touches (FT)
R=hydraulic radius=A / P
S=slope in decimal (FT / FT)

Equation can be changed to solve for the cross section area needed to pass a certain Q value.