Bracelet sizes are measured inside the bracelet, completely around and across the opening. If the reading on the tape measure shows 7", for example, that is the size we assign to it.
Find your bracelet size by wrapping a flexible measuring tape completely around your wrist so it fits like a bracelet. The reading on the tape will indicate your bracelet size.
Most bracelets can be opened up about a 1/4", or closed down about a 1/4" in order to get a good fit unless they are bracelets with inlaid turquoise or other gem stones or similar solid construction which cannot be opened or closed in any amount without damage to the bracelet and/or gem stones.