what kind of sail size do I need as a beginner in 20 - 25kt winds on a JP Funster 205 to get up and going?

Probably wait for the wind to drop to 15 knots.....and the try a 4.5 metre sail thereabouts.
Might be best to go get some lessons .......this will reduce the wear and tear on your body.

i'm floating around the 92kg mark, does this change sail choice?

You are on a great board to learn on.
It should float you fine. The sail should be small just so you can cruise around and familiarize with the power of the wind. Your 92 kg is adequately compensated by the buoyancy of the board.
Get a couple of lessons though.
Sometimes if you are helped by a close friend who is a really good windsurfer...he may have forgotted all the baby steps leading up to his basic skills. A proper instructor from one of the shops (3?) would be the best port of call.

yeh i have booked in my first lesson this week, however it is obviuosly weather dependant. i'm pretty keen to get out there play about!

thx for your advice, always tricky as a n00b starting off

Good luck Scott.

You might be better with something around six metre but it depends.

Matt is spot on with wind recommendation. Fifteen knots is fairly frequent in the bay as it's roughly what a seabreeze gets to.

Let us know how you get on.