A good range here, but how big should they be? Try this as a start point:Main mooring cleats, 3 to 4% of boat lengthSpring cleats - 2 to 3% of boat length Halyard cleats 15 to 20 times the rope diameterJamming cleats (any type) the right size for the line used.
These are leads for running rigging. If it is fairleads for mooring line you need, try here
Handsome is as handsome does
Wykeham-Martin furling gear looks like - indeed is - a relic from the 19th century. But it works. here
Avast, Belay me lad!
Traditional ash or galvanised steel. The dimension quoted is the inner diameter, so you need 1 or 2 inches clearance.
You can specify a horse (or traveler in US English) link.
Useful not only for mainmast track, but also for clew outhauls, and hatch slides