Lots of stuff is going to affect the "ideal" width. Main things are the height and weight of the rider but you also have to consider the effects of rail shape, board volume, length, bottom shape, outline, etc.
I'm 6' and 180lbs, I find that my 150x46 cm boards with concave bottom and very thin (9mm) is just small enough that I can still do all the tricks I do on a smaller board, though some things take a bit more effort.
I tried 150x51 with a thin rails, flat bottom and I found it quite a bit more difficult both for loading the edge for whip jumps and for getting effective no-whip "pop"
pictures of a few of my boards are at: http://gallery.kiteforum.com/gallery/vi ... me=albup46
Volume and rail shape are going to play a big role. The more volume in the board and the softer the bottom edge on the rail, the harder it is going to be to force the edge into the water once you build up some speed and apparent wind power.
With a shorter board you should be able to get away with just a little bit wider because you can more effectively "put the brakes on" by moving your weight back over the tail.
Just my opinions. Also I'm talking about twin tip boards with straps, with a strapless board you could theoretically go as wide as you want, because you can always get more leverage by moving your feet further back and closer to the rail.