Which cleat length is right for me?
Here's a simple breakdown: You need 1" of cleat length for every 1/16" of dock line diameter, so a 4" cleat will suit standard 3/8" dock line. If you're unsure, or need to tie down a variety of vessels, go a size or two larger than you expect.
Sizing a boat cleat is not too difficult if we follow a short chain of dependencies: the size of cleat you should use depends on the size of line you use; the size of line you use depends on the size of boat you have. The general rule is fairly simple:
Cleat length from tip to tip should be 1" for every 1/16" of line diameter and dock lines should be 1/8" of diameter for every 9 feet of boat length.