Depending on weight and board size people tend to have a 7=7.5m or for light wind (a few have something around 8m)
for medium wind a 6=6.5m
for stonger wind round a 5.5
for 25knots plus some go to a around a 4.5m other hang on to a 5.5m

At 73 kilo's I wouldn't go bigger than 6.5 especially as you are trying to learn to water start. The shorter the boom the better to get them .That's the biggest I run and I'm 85 kilos.
Your board is a great size thou...

Discuss: Just read your post on the general forum. Get a powerful 6.0 freeride sail that has a big scoopy "bagged out" shape on the bottom end of the sail. A 6m (just get the 6m only and take it from there) should do you in all situations. Since you are a beginner you will not be efficient, so a 6m would be equivalent to my 5.5. I don't want to brand drop, but my 5.5 is a Severne Gator (every brand will have a Gator equivalent in their range). It's small but bagged out and has an enormous wind range for a freeride sail. This sail is perfect for my big freeride board. Somebody in the general forum post suggested Hot Sails Superfreaks. Do not get these sails (from personal experience). They are flat and too twitchy for lakes (perfect for seabreeze sailing to gybe and try tricks on WITH SWELL. No good for gusty/choppy crap you get at Boggy/Hindmarsh).

Yes, the big powerful freeride sails have a big wind range, a much bigger range than in the old days.

Not "baggy sails" (as in the old dacron sails). "Bagged out" meaning they have a big shape and hold that shape with battens. That big shape will let go (filter out) gusts, make pressure through your arms/harness line constant and the sail easier to control. If you get a flat sail, it will be twitchy, gutless, make your body tired, and your progression will be slow.

Yes, a 5.5 will be too small for you. A 6.0 (my suggestion) or 6.5 (JB's suggestion) will be a good choice.

Just a few points to check when buying large powerful camless sails in my experience:

Check the mast sleeve pocket of the sail (ie: where the boom connects to the mast through the sail sleeve). Some manufacturers (ie: Severne) make this too high on some models. This makes the sail unusable for short people. I had to take one of my sails to a sail maker to get it altered because of this reason.

Check the sail for it's symmetry: When sailing on each tack, the sail (the bottom part of the sail) must look the same. I used to own a big camless sail that was more powerful on one tack than the other (the bottom end difference was a few knots), because the bottom edge of the sail (of the bottom batten) would stick out on one of the tacks making the sail less efficient on that tack. This was annoying in marginal conditions when I would blast out in one direction (in the ocean), and slug in the other. This has an application for speed sailing, but has no place for freeride/bump and jump. As a beginner, you do not want this feature.