Hey guys, this question has probably been asked 100 times before.
Anyway what size board do I go for.
purely for surf, head high and smaller, reef break.
I weigh 90 kgs and have a lot of surf experience and a bit of sup in river experience
I want to get my wave count in,so prefer to sacrifice a bit of performance for a little more size
cheers 
Welcome to the dark side!
Do you have friends with SUPs? Have a play on something big like 10' x 32" just to give it a go, but then see if you can get hold of something around 9'6 x 30". With your surf experience you can probably handle something smaller than we might recommend for a complete newbie. If you get out frequently you will be ready to go to something a bit more performance oriented if you see the need, or buy something of a similar size.
I like my Sunova Speeed 8'10" but that's not where you should start (I am a bit heavier than you).
Also, what are the conditions like where you surf? The amount of chop/water movement affects your ability to stand and paddle hard when catching the wave.
Even as a surfer, SUP will make you a beginner again. SO do not underestimate the new techniques to learn. Practice a lot on the flat in between sessions to rehearse what went wrong.
See for instance: www.seabreeze.com.au/forums/Stand-Up-Paddle/SUP/New-Supper-old-surfer/
This said, once the basics learned, which can take time, your surfing experience will help you a lot.
As other said, you should not go for less than 9'x30", knowing that the planshape will contribute to the stability. 9'x30" would be OK for a shape with wide nose & tail, but with pulled in nose and tail, aim for 9'6"x32" at least.
Length is important for mastering the "row effect", width for the stability.
What you can get less of, as a surfer, is the volume. You can get away with 140 liters, whereas a non-surfer of your weight should aim for 160+ liters
