Board size is very, very much a question of personal preference, even in light wind. But the perceived size also depend on a lot of other things than shear volume (as soon as you are at all moving).
Some popular production and custom brands have for the last few year adapted shapes which are stacked with a lot to extra volume but in my opinion still kind of suck for schlogging since they are still rather unstable. Again, as long as you are at all moving a flatter deck and slighlty fuller (and sharper) rail shape will in my experience help much more for super light wind sailing, despite less volume.
Every time I'm on Maui I sail super light wind and I tend to get by on 75 liter boards (at 72 kilos), albeit it can be a bit of a struggle in the lightest and biggest days.
So after that background.... The question is what it is that you need from your big board. I can see at least three things.
1. shear float for when you're absolutely standing still.
2. Effective slogging at far below planing but still generally moving a bit
3. Effective planing for sailing just around the planing threshold where the ability to pump and get planing to get out of trouble can be a blessing.
For 1 you NEED volume. At or below 80 and you will sink at standstill.
For 2 I reckon something like a Quad 81/82 will do. If your 76 liter boards is anything like I describe above (fx 56cm or narrower) I think you will find the difference up to the 81/82 is quite big.
For 3 I'd say that even the 76/77 will do as long as sails are 5.3 or below. With bigger sails, 81/82 can get a slight planing advantage.
If you want to play safe, you can go for the 87. If riding speeds get really high, it might be a bit more of struggle, but it is still a very easy and comfy board to ride. But I just wanted to point out some options...