If it's blow-through, it can be any size that works with the engine N/A - the extra flow comes from the increased density. Roughly RPM × displacement ÷ 3456.
If it's draw-through, it has to be large enough to supply the higher power level. RPM × displacement ÷ 3456, then × by the pressure ratio.
Assuming 14.7 as ambient:
5 psi = (14.7 + 5) ÷ 14.7 = 1.34
10 psi = (14.7 + 10) ÷ 14.7 = 1.68
15 psi = (14.7 + 15) ÷ 14.7 = 2.02
