Hi there I have a 95 ltr all terrain and love it,It gets up and going early,is very comfortable to ride and when you find the sweet spot it gybes like a dream.Also being a Naish it is also a very fast board.
I should have said also that I am 82 kgs and use sails from 4.7 through to 6.5.The good thing with them is you can use either the inboard foot straps or for flatter water shift them out.
Stefan, you might find that a bit too much board in BF7-8 (27-40 kts). I've got an 85L RRD FSW and am 82kg and have used it up to BF7 with a 4.2m. At 72kg maybe consider something in the 77-80L bracket. What board do you have for < BF6?
the 85 should be good down to a 4.7 for your weight, (gotta remember that you light weights are using a 4.7 when im still on a 5.3) oh and they are really quick, the more you put in the more you get out and wicked for gybing