definitely get a 16 if you want to ride a normal sized twin tip.
Sure you can do laps on a surfboard and 12m when it's light, or you can fork out for a bigger board or make a plywood monstrosity (both of which you'll be unable to ride once it hits 15-20kts). but I'd go for a bigger kite and the board you already have, especially when decent boards are going to cost nearly as much as a second hand kite.
A good 06/07 16m won't cost the earth and are still bloody fun in light winds - sure they turn slower, but then that can be an advantage when you are learning basic tricks like fronts and backs.
I'm 85-90 kilos depending on seasonal variations (christmas = 90kg...) and have ridden a 16m/12m/9m combo since day one and would still be riding my 06 16m vegas except for the new rebels i got for christmas.
I'm with mrjf - don't listen to the lightweights from another state.
young fools...

now, about my 16m vegas for sale - yeah i'm a shameless pimp, i know
www.seabreeze.com.au/Members/Classifieds/Gear/2006-North-Vegas-And-Rhino-16M-9M-16-metre.aspx?id=c_68