You measure it across the case excluding the crown with a ruler or calipers. (See picture. The size is 43mm)
Or with a caliper (note the 0 mark..again 43mm)
Note: I use plastic calipers to measure, while not as accurate as the metal digital ones they are less likely to scratch the watch.
Just to be complete for future reference some other measurements that will be helpful down the road. Doing this also for future reference.
Lug width is important to determine what strap, bracelet or spring bar size you need.
Here it is being done with both a ruler and calipers. This watch has a lug width of 21mm:
Little be harder with a ruler:
Last one of the measurements that once you know your limits can help you in choosing the right size watch is the lug to lug measurement (length).
Here the watch is 51.5mm lug to lug:
Case thickness is also good to know if your trying to fit a watch under a shirt cuff (thinner generally better) or are looking for something with more wrist presence with out going overboard. While it can be done with a ruler a caliper is easier. Below the watch is just under 12mm thick. While not ultra thin this would be considered a reasonable size to fit under your shirt cuff. Again this would depend on your physical size. I can fit most 14mm-15mm under my shirt cuffs.