The average socket cap screw head is around 1.5 times the diameter of the screw shank, and about as high as the shank is wide. However, alternative head designs for socket head cap screws include low head, button head and flat head varieties, intended for driving into countersunk holes and other application-specific mounting positions.
When looking to buy socket screws, note that the stated length of the product is typically measured from immediately under the head to the tip. This is the case for most screw types, but some specific varieties - thumb screws, for example - will often give stated length measurements that include the height of the head in the overall specification.
Most socket screw types are available in either metric or imperial measurements for length and diameter, depending on the particular manufacturer or supplier in question. Many leading brands and socket screw manufacturers will often provide a socket screw sizing chart or similar tool that can be referred to at the point of purchase.
While relatively small socket screw sizes (of diameters ranging from M2 to M6, and lengths of around 3mm to 20mm) tend to be more in demand across a wider range of tasks and applications, the full range of available sizes is considerable. Larger, heavier gauge and extra-long socket screws are not at all uncommon, particularly in widths of around M8 to M20, and lengths of anywhere up to 130mm.
Extra-long socket screws are often found in electrical plug sockets. Common electrical socket screw sizes include 40mm, 50mm, 60mm, 75mm and 100mm.