Very much dependant on your type of sailing Abby. In the parts of Europe where marina pontoons are the norm 2 or 3 lines of 1.5 x boat length plus a couple of shorter say boat length all about 12mm or 14mm and stretchy [octoplait nylon works well] . In addition one really long [as long as you can stow] 10 or 12mm to get you out of bother without joining bits. In the med and where stern to the quay is done much the same but need at least 2 really long [to tie round trees and rocks] again the stretchier the better. Geoff