I agree that it's the conditions, not the board. Sure a 7.0 will work fine, but if you are using it in 20+ knots, and likely rough water, it will be a bumpy ride. If you are in protected water and the chop is small, no problem.
I frequently use a 6.6 on my iS 111, and it works great, but it's the chop and pounding that soon drives me to a 105 liter bump and jump board with a smoother ride.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.