it all depends on the winds.
I have two friends with 115 kg...and they mainly ride a 16 or 20 sqm kite.
For the beginning a 12 is good to not be overpowered, but this won't get you going in normal wind...you need about 20 knots for your 12, if not even more.
The secret will be the board in my opinion. Get something like "The Door" from http://www.spleene.com
It is a big board, but gets you going, even in lighter winds with a smaller kite.
So, start with a board like this, and once you can handle the power of the kite etc, take a bigger kite and a smaller board. But do keep the big board for the lighter wind days.
Good luck, and keep us informed about your choices and how you progress...if you have a blog, it would be great, since there are some heavy guys out there who would like to know as well, what works and what not!