One of the difficulties of buying a purse over the Internet is not knowing in advance how long the strap or straps are. The overall length of the strap determines the "drop," which is the distance between the middle of the strap (where the purse lands on your shoulder) and the end of the strap, usually located at the top of the bag's body. The length of the drop determines both how the purse is worn, and as a result, says a lot about the style of the bag.
Hang the bag from the center of the strap(s). If you are seeking a handbag whose strap is the same length as a bag you already own, hang the bag on a doorknob.
Measure the drop. Using a tape measure or ruler, count the number of inches from where the strap hangs on the doorknob to the top of the bag's body. That is the "drop."
Measure the strap. To measure the entire length of the strap, double the result from Step 2.
Estimate the drop. If you want a handbag to fall at a certain place on your body, measure from the top of your shoulder to the point on your body where you want the top of the bag's body to fall. This is the estimated drop. Double the estimated drop to estimate total strap length.