Watchmaker's hammers are sold by size, with the most popular being 60, 70,80 and 90. This measurement refers to the hammer head length in millimetres. Here is a small curiosity: since the 1980s watchmaker's hammers have been no longer made in Switzerland, so even those signed by Swiss toolmaker brands are actually made in the capital of hammers: Lahore, Pakistan.
I use my hammers every day: 90 for removing bracelet pins, 70 and 80 with a staking set, and 60 for riveting the finest watch components like balance staffs. Absolutely an essential tool in any precision workshop.