Yo, went out windsurfing, the other day. I have a 145l board and a 7.0 metre sail fin is pretty big like probably around 40cm. It was windy, around 25+.
Got planing, all sweet, fin seemed to lose grip on the water at really high speeds though, meant the board started planing sideways with tailslide. I seemed to get even faster when this happened.
WHY?? it was creeping me out hell bad. Someone help me with a solution, smaller fin? smaller sail?? smaller board?? i weigh 70kegs, it was on flat water.
Board manufacturers usualy provide the fin specs for their boards. I would check your board home site.
As for me, it happens when I put on a fin which is smaller than the manufacturer's spec.
My board is 160L and is set up with a 52cm fin.
Yours should be...48cm maybe?
Spinout 
Many reasons why it can happen including too much pressure on the back foot for the fin to handle, a bad fin, doing a jump and the nose being upwind of the tail which can especially happen when overpowered in choppy water.
What to do about it? Learn to recover when it happens by pulling the tail upwind of the nose. Its all part of the learning experience and it can happen to most sailors.
Yes, its spinout, more often than not caused by too much weight on your back foot / riding on the fin too much when going upwind.
To correct it get all your weight down through the mastfoot, kick the nose of th board downwind with your front while scissoring the back under your arse! Be very careful doing that at speed though so as to not catch the nose of the board in chop or a catapult (broken boom in my case) is likely.
If it is a regular thing then you need to change your stance and not weight the back foot so much. This could mean that your harness lines need moving back a bit, as when they are too far forward you are pulling too much with your back hand - and so weighting your back foot to do this. Someone told me to move mine back and it stopped spin-outs almost totally when I was learning (apart from on one particular fin....)
All the above is right about spinout, but strewth if it was actually 25kn and you were on 145L and 7.0 I don't care what fin you have it will be hard to handle.
In consistent 25kn, at 90kg I am comfy on 80L and 5.0. Fair enough that is wave gear and race sails will handle more wind, but really u are at the limits of the ideal wind range for that setup (even if you are heavy) so the most minor technique errors will be magnified greatly
