I used to sail similar boards. RRD 281 in '97 - '98 and RRD 278 from '99 to '05. I assume your board is somewhat the like and advice accordingly.
The 281 was a real beast, 54 wide, ultra efficient shape with thick rails in the tail and a short flat section. The 278 was a bit tamed (the shapers told me Anders Bringdal asked for that, as the 281 was too demamanding and forced him to concentrate on the ride instead of the race...), 55 wide, more curve in the outline, stretched flat section and lower nose, thinner rails with more tuck. Both fell in the groove of classic "nine footers" medium wind (by old standards) slalom boards, the 281 a bit biased towards lighter weather.
Such boards are very hard to sail compared to modern designs, but deliver incredible performances, althought in a narrow range. If you master those, you'll raise your sailing abilities by heaps, and this will prove when changing to modern designs.
Expect to be able to "actively" plane in light winds, expecially on flat water. By 18 knots you could be the faster out there in flat waters on a beam reach, by 20 - 22 you'll be screaming, by 25 in chop or waves you'll be screaming for different reasons. Tame the beast and make it to 30, it's rewarding, even if not easy. On such boards, sailing ability and fitness do much more than on modern designs.
A nice 6.6 - 7.0 will pobably have the widest range on that. The narrow tail will not let you use a big fin, so probably you'll have to rig flatter than usual. A smaller sail will help some in high winds, but don't expect much, as your problem will be to keep (some of) the board on the water, not to sheet in. Probably a 6.0 - 6.3 will be the smallest useful. Yes sail size range is probably that narrow, as is wind force range.
DO NOT expect a "de-tuned" sail to do the job just because the board is old. Such rockets are extremely demanding. On beam reaches the speed is so high the leading edge will easily collapse if not held in place by cambers and luff tension.
As said, fins have to be small and softish. Old elliptical Deboichets (todays' UpWind?) in 30 and 32 did the job for me, with 6.6 and 6.9 sails. More powerful designs such as old Tectonics Spitfires 31 and 33 didn't do well, as the power in the tip was useless with such narrow tails and soon out of control, and just slowed down the ride. In recent fins, I'd try some soft designs such as Tectonics Falcon (Vector EX, MFC SL... ?) in 30 to 34, or maybe slightly more powerful Tectonics Goldwing (Vector Rockit?) in 34 to squeeze some power out of the small board tail.