Diameter for 32ft ~ 16mm
Bow and stern lines ~ 2/3 of boat length: 21-22 ft.
Spring lines ~ same length as boat: 32 ft.
Ropeinc site above suggests that 12-strand is not as strong as double braid, and that coloured line is not as strong as white. Nylon is good because it has stretch.
**2019-12-16: bought 4 x 16mm, 10m lines. Should have got two of them half as long again, because our boat doesn't have midship cleats... so the springers have to lead back to the fore and aft.**
from Animated Knots
Breast Lines prevent rotation and should run roughly at 90o to the dock. To gain length, they should be led from the farthest part of the boat: the bow itself (or the outer hull of a catamaran) and from the far quarter of the stern.
Spring Lines prevent fore and aft movement and should run nearly parallel to the dock and may cross each other to obtain an optimal lie.
Double all lines with high winds (50 kt +)
The 'cleat' on our foredeck is actually called a Samson post, although perhaps not a true one? Guess it's because needs to be strong to hold the anchor line?
Seems like midship cleats might be worth it.