Well considering this was a 12mtr and I was going upwind easily, I'd say for a foil anything larger than 12 or so is not really necessary,
However, I am considering a larger foil kite just to push the boundaries a bit further.
Plus there is still the old school big air enthusiasts.

My latest kite purchase though was a 3 mtr single strut kite for use with the foil on 20+ knot days.
I would never have dreamt of a 3mtr pre-foil days, the 5 only got used a few times a year.