Bille, you of all should know not to use units that is only used very locally.... (at least not without WRITING the unit)
Wind is measured "globally" and international with two units only:
For sailing and historically, knots is used, and well known everywhere in the world without question.
And the metrical unit is meter per second (m/s) which is used in europe most often, in fact almost always here, instead of knots.
1 m/s = 1.9438 knots
1 knot = 0.5144 m/s
So close to a factor 2 in difference, which our "minds" can easily reckognize and understand - which is good - otherwise it would be impossible to understand what others are talking about....
Every other unit, mph (or even the horrible beaufort scale) are not really used or known anywhere, except the local country specific uses
Having a 4.5m2 kite in 35-40 knots would fit just fine, not even overpowered, for the average 80kg rider, so no problemo and Jan is spot on IMO
4m2 kites are used relatively often in waves, and some (often lighter girls) got a 3m2 for those gnarly days with huge waves where they just WANT to get out !
A 3m2 can be used up to about 50 knots without being overpowered, in theory...
In real life, when it is over 40, it becomes very fickle, because of the gusts as said, but also because of the huge line slack, and the impact on the board and rider
More important - the water gets "skunked" and horrible, because of windchop - so not really good on flat water nor in waves
On the other hand - you can have some AWESOME sessions with 4 to 5m2 kites very often, both TT and Waves, so I think it is very different what different kitesurfers think about it....