Go 6m. Once there is real wind, you can drop kite sizes really quickly.
I have 12, 9, 7, and 5m and the 9 and 7m have way too much overlap. A 5m is often too powerful when it is over 25 knots depending on the foil.
I reckon the ideal foiling kite quiver would be 12, 9, 6, 4m.