They varied from nation to nation, sail to sail, and era to era. Modern rope materials require smaller diameter reef points than the rope of the 18th century. Reef points varied in length depending on the size of the yard in times past. According to Falconer's Dictionary of the Marine the lengths of the reef points are nearly double the circumference of the yard to which their sail is attached. The thickness of the points was tapered but not noticeable at our common scales. From a post here at SoS....
A reef-point is a short braided rope / line hanging on both sides of the sail.
The length of a reef-point is the same on both sides of the sail.
The reef-points are fitted through a series of eyelets (reef-eyelets) located along each reef-band (1, 2 or 3) across the width of the sail.
Once the rope is passed through the eyelet, a knot is tied on both sides of the sail cloth / eyelet to secure the reef-point.
Depending on the nation and era of your model the drawings of reef points of the 18th and 19th centuries on page 162 of James Lees' The Masting and Rigging of English Ships of War may be helpful.
Allan