Boats the size of Valdivia and Pride often are referred to by Length-on-deck - which, basically, is the longest length a deck plank can lay flat on beams.
Length on waterline is a common reference. Pride was 79' on the waterline (with the rudder), that's 79*12/24=39.5" in 1:24 or 47.4" in 1:20th. She was 90' on deck so the rake of her stem and stern added 11 feet.
Long overhanging ends vs short ends and inverted transoms, etc can make waterline vs deck length kind of odd to deal with when comparing boat sizes.
Scale is scale regardless of measurement - 1/20 of a meter is the same as 1/20th of 39 inches (or whatever a meter equates in inches).
As for model size, I subscribe to the idea that the biggest model you can transport and launch is best. Bigger sails better and more realistically, it's easier to see what it's doing out there, and it commands a lot more "Wow!" from observers.