There is nothing more distressing to a watch aficionado than spying a poorly sized or un-sized watch bracelet. To me, it is as almost akin to fingernails being run down a chalkboard, LOL. I feel for the unfortunate timepiece, made to flop needlessly about the wrist.
For the uninitiated, there are benefits to properly sizing your watch bracelet. The first benefit, is that a properly sized watch bracelet has less of a chance of leaving your wrist, intentionally or by accident. Keeping the watch stationed firmly atop your wrist will also prevent the case of the watch from damaged by accidentally coming in contact with objects and surfaces such a tabletops, doors, etc., not to mention, minimizing the bracelet itself from unnecessary contact, save the usual desk-diving marks that will happen on the clasp enclosure through daily wear.
Most jewelers will size standard watch bracelets from between $10 - $20 dollars, but price will vary based on the complexity of the bracelet design and the type of pins or screws used in the construction of the bracelet. There are some excellent sizing kits available on the market that will allow you to size your own bracelet with minimal effort, and save you a few trips to, and dollars spent at your local jeweler. The sizing kit pictured below from watch company Croton was had for less than $20 shipped from vendor www.talkaboutwatches.com. Keep in mind, a perfectly-sized bracelet should allow enough room to slide you index finger between your wrist and the bracelet; cutting off circulation to your wrist should be prevented.
As with any clothing accessory, comfort is key, and personal taste is the most important factor.