RE: 90kg and 7.5...what size board?
iS122 will be a good safe choice for 90kg 7.5m.
If it's a solid true 13-18 kts, iS111 also being very suitable and even iS101 possible.
A key factor in the choice will be other sail sizes and/or which end of that range you will spend most of your sailing time in. In that wind range, personal sailing/tuning style/experience also plays a role, as some riders prefer the ride/handling of a smaller board offset against a higher technical/skill/energy input vs others who choose a more "automatic" performance with a bigger board at the expense of a little top end potential..
Local water conditions (severity of chop) will also play an important role in board /size choice for the conditions (some riders prefering the calmer, less bouncy ride of the smaller board which tends to sit down more into the chop, others preferring the "blaze across the top" style of the bigger boards. Both extremes are effective, but to be most effective, board choice should be matched or compliment rider's style/technique/preference.
Please let us know if you would like more comparitive information.
Cheers ~ Ian