There are several different systems used to measure the size of a watch movement and which one to use will depend on who you talk to. No matter what system you use, the larger the number, the larger the watch.
Most pocket watches are described using this system (ex. 18s, 16s, 10/0s). The Lancashire gauge is based on the 0s being 1 inch in diameter, but an additional 5/30″ was added for the “fall”, except when the size is over 16s, in which case 6/30″ was added for the “fall”, except when the larger watch was designed after around 1910 in which case only 5/30″ fall was added. The “fall” is the amount added to the dial plate (bottom plate) to form a flange to keep the watch from falling out of the case. Once watches got smaller than 0s, they first used the notation of “00s” and “000s”, but that quickly became awkward, so they changed to using “2/0s” and “3/0s” instead. This is why you will never see a watch size of 1/0s, and why a 3/0s watch is only 2 sizes smaller than a 0s watch. Wrist watches tend to range from 6s down to 26/0s for the smallest lady’s wrist watches. Learn More
This system is used by most modern watch companies to describe their watches.
The Swiss used a different system based on the “Ligne” (which is French for “line” and pronounced “line”). One Ligne is 2.2558291mm or 0.088812168 inches. Like the Lancashire gauge, this is an old system but it makes a little more sense than the Lancashire gauge. There are 12 Ligne to one French inch (pouce, French for thumb) and 12 pouce to a French foot (pied).