Well... recently, I would have said "you need a big board" too but after a month in NZ enjoying demos on Axis boards, I have to say that the right board is... just the right board.
Basically, if I couldn't go upwind on the smaller boards, there simply wasn't the power in the kite to do anything -- even if I swapped it for a bigger light wind board, I'd just be going upwind a little and mowing the lawn. Incredibly impressive, the Vanguard and Limited boards; I loved them so much I bought one
I had a Nitro3 14 for 2 weeks too... weather was never quite right; either way over- or just under-powered and I only got to ride it "in range" for about 10 minutes
Mention it because it was really good at the bottom end... very little in it between that and my gen1 OR Flite 17.5 - I think cos the Nitro make such good power easily when worked and was faster in the turns. I think maybe a 19m kite would have been better but there was seriously very little wind.
I can see how people would only have a 14 as their biggest kite and just not bother when the wind was so light... but I'd never last through summer without my 21