When custom-ordering a bracelet, it is crucial that your submitted measurements be accurate. The measuring process is two-fold. There is the actual wrist measurement, and then there's the knuckle clearance measurement, so that the safety chain is made long enough for the bracelet to go over your hand.
You'll want your Renate Kasper bracelet to fit more like a watch and less like a bangle. Do you know how you check a pet collar by making sure you can fit a couple fingers underneath? Well, a proper fit with your bracelet should allow you to easily insert your little finger underneath (to about the second joint) without popping the magnetic clasp.
Make your two measurements with a strip of cardstock, posterboard or cut it from something like a cereal box. A piece of string will not give you a correct measurement. Cut your strip to be approximately the same width as the bracelet you want to order. If using a cereal box, try to get a strip with no folded corners or kinks in it. Make sure your strip is long enough to encircle your hand, plus a little more. (I recommend checking your hand diameter first.) Cup your hand as you would when putting on a snug bracelet, and then wrap the cardboard strip around the knuckles at the widest part. This fit can be tight. You won't need to insert your pinky to check this one. Once the strip is overlapped on itself and cinched up to the correct diameter around your hand, mark the strip, and measure with a ruler. Next, when sizing your wrist, attach the cardboard strip in a similar fashion, making sure it's fairly snug. Check it by doing the pinky test. Wear the cardboard band a little while. Does it flop around? Is it too tight? You don't want it to spin around very much, but leave yourself a little clearance. When you get it adjusted to a fit you're happy with, instead of taking the tape apart, simply cut the strip crosswise at any point along the band that isn't taped, and measure the length of the strip.