It depends a lot on whether you want to compete on the board, or just have fun.
For competition, if you are well powered up on a 6.0 at your weight you would be far better going on a smaller iSonic. For fun, if you are not max powered a 6.0 can do the job. 5.5 is just too small to keep the board on the water. I (193 cm 95 kg) sail my iSonic 122 with a 6.5 (solid no cam freeride sail) in quite a lot of wind and the board works surprisingly well with this sail size. I'm not at all in the mood to be on a smaller board. For competition use I - and you - would be better with a 7.0 cammed racesail though.