In deciding what size belaying pins to use on your model there are some general rules to follow. Most ships used one size of pin with the largest required size prevailing. The diameters of pins were generally not less than the diameter of the ropes belayed to them. Sea-going naval and commercial vessels had pins from 1 to 11/2 inches in diameter with lengths of 12 to 18 inches. Smaller vessels could have pins of 1/2 in. diameter and lengths of 6 in. A general rule of thumb was 1/2in. + 1/2in. per 100ft. of ship length for the diameters and 6in. + 6in. per 100ft. of ship length. Pin shapes varied slightly but all had rounded ends, shoulders on the upper portions and a slight taper to the shaft. Upper portions were generally between 3/8 and 1/3 the length of the pin.
 

For example:
BELAYING PIN
PROPORTIONS
• 30' ketch- length 7.75 inches, diameter 1/2 inch.
• 160' schooner - length 15 inches, diameter 1.25 inches.
• 200' ship - length 18 inches, diameter 1.5 inches.