At 170 American lbs, my quiver sort of sets up like this with relation to wind speed:
5.3 Less than 20mph average
4.7 20-24 mph average
4.2 25-29 mph average
3.7 30 - ∞ mph average
I think I'm like most windsurfers in that I don't go down a past a certain sail size. At some point, if you're a passionate windsurfer, you're going to encounter some gusts in your career that are over 40 knots. Most of my sails I keep within a 5 knot range of one another, but I've given my 3.7 the unenviable task of having a range of at least 15 knots before I completely throw up my hands and give up. Usually those "windy days" pay no heed to your sail sizes and the average could be anywhere in the 30s and the gusts will happily roll through in the 40s.
Yesterday, a wind blew through the bay that had the sensibilities to realize that I wanted to be a happy windsurfer on my 3.7. Instead of being the usual DEAR LORD HELP ME iddy biddy sail weather, it was perfection from 2:30-4:30. The settled wind was confidence inspiring, so I was happily jumping, going for vulcans, and even going for loops. This was not a typical 3.7 day, at all. It was a great one!