In 30 knots average wind on a waveboard for average weights, a 5 m2 starts getting too big pretty fast, and 4 and 4½ m2 starts to fit very soon.
50 knots as said, is simply a very wrong interpretation of how much wind there really is, and I bet 99 % are wrong when they say they were out in 50 knots (average wind, only thing that counts...) with big kites 5-6-7 m2 on surfboards.
When windy, 25 knots and up, meaning 6 m2 kites and smaller for average weights, some uses 18 m lines as you get a much faster redirect, and can control it easier in gusts
Myself, I dont like the shorter lines, as I find them to be too much on/off and not give smooth power and control - but this is a thing only you yourself can learn, trying - listening to us will give you no clue whether you will like it or not, sorry to say or6.
I have no idea about TTs, but you only asked about surfboards/waveboards as I understood your question?