I own an IS133 (85cm wide) and IS110 (75cm wide) an IS101 (64cm wide) and an IS87 (56cm wide)
The equation is not just about width or about volume, but a combination of the 2. I weigh 88KG, The Is133 (which is actually 127L) is a great board, however, with my weight and a 9.6 it is actually quite difficult to get planing, and requires a good technique. However, once planing it is brilliant and seems to power through every lull. I would say that the IS127 is ideal as the biggest board for someone around 80 KG. I have not used the 137, however, I would think that it will get unmanageable for lighter riders very quickly.
If your only intention is only to race, the IS117 is perfect for a most sailors as it will be working well at the minimum wind speed with a 9.2m sail. Heavier sailors will probably use the IS127 with a 9.6
If you want to maximise your time on the water in light winds, I would go bigger and have a IS127 or an IS137 for heavier sailors
I will be changing my IS133 for a bigger board. I find that the IS133 is too close to the IS110. The sweet spot on the IS133 is the 8.6m sail - It handles the 9.6 well, I am just feeling that with my weight it needs a few more Litres. The Is110 also handles the 8.6 extremely well, so as soon as I am powered up on the 8.6 then I will be on the Is110 as I prefer that board
The Is110 is an amazing board, it has a wind range from about 14kts on the 8.6 up to 25kts on the 7.0. It is a quick board, however, it feels very controllable even on very choppy water