Rocker, volume distribution, tail design (is it boxy or rounded, cut outs or not, a lot more factors than width influence), fin design (not only size) there are lots of factors. And finally sail design has changed a lot during the last couple of years. Probably any board wil be happier with a 2008 design than a 2000 design even if the former is a little too big.
Anyway James is right. Oversized sails just feel wrong no matter how you try to control them.. In my experience breaking the recommended limit with about 5-10% is ok, and that's about it. Going bigger will make you plane slower, sail slower and get tired faster.