To determine the length, we recommend using a piece of string or yarn, attached to your bag, and adjusted until the length is just right. Measure the string from end-to-end to determine your strap length. Another method that often works well is to find another handbag you have that has the strap length you're looking to replicate. If possible, attach it to the handbag to see if it will work for the length you'd like to achieve.
Feel free to explore this blog for additional details. How to Choose the Best Bag Strap Length.