Sizing up a twinny Grumps?

I reckon when you're well powered up (doesn't have to be a 4.5m day, can be well powered up on a 5.3/5.5) you can go 10L under your body weight. So if you're 97kg, an 87L board shouldn't be a problem.
Depends where you sail as well...the inside section at Metts is always much less windy than outside the break, and from memory (haven't sailed there for a few years) Scarbs has a bit of a wind shadow as well... it's no fun dogging into a crunchy shorebreak.
The Woodies skate park has pretty clean wind, no drama's there.
My Manta 59 (86L) is 20L under my body weight, but slalom boards tend to have a bit more volume in the tail. Still I can sail it comfortably in 18-22K with a 6.6 (preferably at Woodies...the river is too patchy).
And buying smaller kit is great incentive to drop a few kilos! Not saying you need to of course.