To answer your question -
"But still one question remains.
Why do a lot of wind surfers refer to their used sail size then they talk about conditions for a session then?
Why not just wright that it was blowing 30knots and I had a 5.2 sail. So everybody understand that its blowing 30knots and can refer to that easier, then just wright it was 5.2 condition.
Or is that some secret windsurfer code so no outsiders can understand that why meen?"
No code, just a quick reference to what sail they used. As Roger says, judging wind speed when it is 20 to 30 knots is pretty hard, but if a 5.0 sail worked well for the day, then we just say that it was a 5.0 day. The reality is that some may have been on a 4.2and others on a 6.2, and the wind may have ranged from 15 to 30 knots. Inland lake sailing offers variable winds, it's never steady, just up and down. About two weeks ago, I was out on a "4.5 day", but at times I was slogging (not planing) and at other times I was overpowerd.
In the Dallas, Texas area, iWindsurf.com posts reading from 11 airports and three lakes, so before I go out, I know what the wind is doing. I petty much know what I will rig before I get to the lake.
The anemometer idea fthat Roger suggests is a good idea for beginners or intermediates, but eventually, you will just know what to use. I have two, but I haven't used them in several years.