Hey CMC,
I was in the same boat as you- experienced surfer looking to take on sail boarding, and primarily in the suds. I started off with a 150 lt board, got some of the basics kinda worked out (tacking, slo-mo gybes etc) in the first few months,even used it a few times in the surf, then sold it to fund the next board- a 116 lt FWS style of board. Anyway, this current board is a good manageble size for me, is super quick, floats well so you can up-haul and turns sharp and fast in the surf. I've only been sailing for 6 months.
My theory, don't get too big a sail. Something around 5 meters will be heaps, and well and truely usable when you progress to the next smaller board and hit the surf. Too big equalls hardship. And too small a board equalls the same.
Whilst a good surfing back ground will certainly help speed up the painful learning process (which I'm still well and truely in...), the two sports are way different in terms of feel and and to some degree technique. But having said that what seems impossibly hard will start to fall into place in only a handful of months. Then the fun begins!
Personally, I don't think I'll ever jump back on a surf board again, as the sail board is just so much more exciting, way faster, more powerful and has an endless array of things to learn. The days of fighting over a few decent waves in a crowd are well and truely over. And when there's no swell, you can charge the flatwater. It's a win-win really.
Good luck hunting down a beginners kit

Cheers.