As the railsaver, plug positioning, and cords add at least 1' to the length, you should use 1' less than the board length.
here is a 6' leash on my 6'9"
And a 8' on my 8'9"
I use a 7' on my 8'4" for instance, and 6' leashes up to 7'6" boards. A shorter leash means less danger for surfers around you, a quicker recovery ( board back to you with a leg kick) and less tangles. A longer leash is better in bigger waves to keep the board away from you in wipeouts and dampen more the wave pull.
The difficulty is finding thick enough short leashes for powerful head-high waves. I think only DaKine makes a 8mm 7' leash for instance.