as usual - depends on:
where you sail ie conditions
what shape you are in
can you take lessons
can you rent stuff
i am just under 100 kilos and used a longboard for years ie 200+ liters
first shortboard was a 160 liter /79 cm which i still use in 20 to 40 kph winds with 8.x sails
after that i purchased a 94 cm wide FreeFormula for early planing in light winds, 124 liter for higher winds and a race longboard
my race longboard gets the most usage - because i use it at the chalet lake , local river and in lighter winds
i tried to teach a bunch of dudes - one who was also about 100 kilos, he had difficulties with the FreeFormula AND the longboard
i realized volume is vital - the more the better TO START
if you pass this stage quickly, what to do with the board ??
THAT is why i suggest lessons or rentals at first - narrow it down - pardon the pun
GOOD LUCK, GOOD WINDS
welcome back to the best sport ever

http://joewindsurfer.blogspot.ca/2011/05/heavyweights-and-windsurfing.html