Well I found this thread over on RealSurf
He's 90kg and riding a 5'2" and sits armpit deep in the water! As I suspected you don't need quite as much volume as a more conventional board (no idea of the volume but at a guess mid 20s)...
So it's been finished for a month now and I have ridden it in everything from shin high slop to overhead barrels. here's a bit of a run down of how it's going for me:
Board: 5'2 x 19 x 2 5/16
Me: 5'9 1/2, 90 kgs (200 lbs)
I made this board for a bit of fun after watching curren rip on his and I had a spare second blank floating around. I wasn't exactly sure where in the quiver this board would fit.
First couple of surfs were pretty small waves with shapers KS 5s (KS2.1). first impression was that the board was going to be too small for me, when I sit on the board the water is up to my arm pits. However, although the board doesn't float me very well, it catches waves surprisingly easy. It took a few waves and bogged turns to get my feet in the right position. Once I had that sorted the board felt skatey, but never like it was going to slide out. It takes a bit of work to get it going in tiny waves, like a HPSB, but once the waves are waist high or have a bit of push it lights up and planes really well.