I'm pretty new to windsurfing and need a larger sail, but I don't know how large of a sail to get. I got my rig at an auction so it's all pretty old, with the board and mast being from the 1980's I think. The board is a 230L Mistral competition SST longboard, and the mast is 4.6m but I don't know the brand, only the specs. Right now I have a 4m wave sail that came with the board which works pretty well since I'm only 55kg. Near where I live there's a spot I go that averages about 5-10 knots so I was wondering what size sail would be best, whether it should be cambered etc. I've already done some research and think around 8m would be good, but I would like to know what someone else thinks.
Ideal would be a longboard-oriented 7.5 without cambers - the Ezzy Zephyr comes to mind. On that older design you'll want a longer boom length than would be desirable on a more modern board, so you may be able to save some money if you can find an older 7.5 in decent shape. A 7.5 is likely to rig on that 460 mast where an 8.0 might not, and in any case at 55kg even a 7.0 might be plenty. Look for a boom length of 205cm or more, otherwise the center of effort will be out of synch with the mastfoot placement.
I agree on the 7.5m longboard sail. You might get away with a 7-7.5m freeride foil sail like a Sailworks Flyer.
At 55kg I think about 6.5 would be nice, maybe 7 at a push.
I would agree with the 6.5 while at the "new to windsurfing" stage. Maybe with experience a 7.5. It's the equivalent of an 80kg rider using a 9.5. Plus, in 5-10 winds, one is limited by displacement speed anyway.
A 7.5 m longboard sail is great for your weight if you are fit, like exercise, and want to race. For just cruising around in 5-10 knots wind, you're much better of with a smaller sail. A 5-6 m sail will give a bit more stability and speed. But if your current sail is from the same time as the board, keep in mind that sail designed has changed a lot in the decades afterwards. A 6.0 from the past ~10 years will require a 4.30 mast (maybe even a 400), and not have as much low end power as an older sail. So unless you plan to get a new mast, too, either go with a new-ish 7-7.5 that fits your 460 mast, or get a 6.0 with a vario top, or even get an "ancient" sail around 5.5 m.
I'd go with Boardsurfer's recommendation to find an older sail of 5.5-6m. It will probbly be lighter than your current 4, much lighter than a typical modern 7, and have more low-down power and better balance than a modern sail on a longboard.
You could try to chase down a second-hand Windsurfer LT 5.7, which is pretty similar to the sail the SST had when new (if I recall correctly) in terms of draft. Even a new one is a lot cheaper than a new modern 7.
Longboard sails need different characteristics to shortboard sails. Think of it like a motor with deep-down torque rather than high-revving peak power.