I understand your situation and since you only weigh 72 kg, you don't need the really big stuff.
Any longboard with a dagger will be faster in light winds than the wide boards, but once a wide board begins to plane, they blow by the longboards at all points of sail. I raced longboards with sails up to 10.6 meters for almost 20 years and then converted to formula. However, I rarely freesailed the longboard(s) because I had more fun on 130-150 liter slalom/freeride boards with sails up to 9.2 meters.
What you do depends on two things.
1. Your general conditions and what you want to achieve. If the winds are generally under 12 knots, do you want to maximize planing or do you just want to cruse around and plane when the wind goes over 10 knots?
2. Your skills at the moment and how much time you will spend windsurfing. If you sail often, then your skills will improve relatively fast and you will become accustomed to the higher speeds and will most likely want to maximize your planing time.
Wide boards and big sails feel awkward at first, but when you get comfortable and don't drop your sail very often, then cruising around on 9 meter sail on a wide board feels no different than a 7 meter on a 130 liter board.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.