Mooring cleats are mounted on docks and boats and are used to secure a boat by wrapping a line around the cleat several times, often in a figure 8 pattern. Cleats are often used in conjunction with chocks in tethering a boat.
Generally, manufacturers recommend just slightly under one inch of cleat for every 1/16-inch of line diameter , which means you need a six-inch cleat for 3/8-inch line, an eight-inch cleat for 1/2-inch line, and a 10-inch cleat for 5/8-inch line.
Full dimensions are on my website - Mooring Cleats
Fig 3 shows different sized cleats and what they would measure in various scales
Fig 4 shows dimensions of all of the cleats in mm, with the chart referring back to Fig. 1
At the bottom of the page are links to all of the current cleats on Shapeways