You will get a lot of use out of the iS137. For an XL slalom board, it has a massive range, but you must invest in 3 fins. I'm slightly heavier at 100kg and use a 52cm, I only use this in super light winds, 8-12k winds with a 9.5m. 48cm is also use with 9.5m and 8.6m, and when its a solid 15/20k I use a 46cm with the 8.6m. Iv'e even used the iS137/46cm with a 7.8m in 25/30k winds (it was all I had with me that day) and it worked really good.
What I'm trying to say is, with some decent fins, you will get a big wind range from the iS137. This would mean it is pointless getting an iS122.
At some point, your next size down has "got to" be much smaller than the iS137. For you, the iS107 or FU111 would be best. At 100kg, I do not sink on my iS107, maybe up to my ankles if the wind dies completly. You can uphaul on the iS107 if your quick about it. The iS107 is still a big board, so do not worry that it will be to small for you