If you haven’t had a chance to try on a watch of interest, but want a sense of the size, there are a number of sizing guides available online. With these guides you can print the page on 8.5″ x 11″ paper and see the size of the watch and strap or bracelet. And if you already have a watch, there will be a section on band width, which is a measurement of the width between the lugs to determine what size strap or bracelet is required.
One good examples is Crown & Caliber’s Watch Sizing Guide.
If the watch is on the way, and you already have the measurements, you can start planning for what strap you might want to swap to using some sites tools, such as the Hodinkee Strap Finder (the number of watches is quite limited, but can help give a sense for some different styles.
There are a number of “how-to” guides if you want to change straps (and bracelets) yourself. Really not too hard once you get the hang of it, but we would advise practicing on an inexpensive watch just in case you scratch it up!
How to charge a watch strap/bracelet:
– How to change an Omega bracelet to a strap