What length?
How do I measure?
Here's a couple easy ways to determine your strap length:

    *If you have a strap simply measure it from where it attaches to
    the endpin to the end of the strap (the part that is inside the
    scroll/bottom or headstock or strap button). Add or subtract
    whatever you may want to make it more comfortable.

    *If you don't have a strap, simply stand with the mando at a
    comfortable position and measure from the endpin to the TOP of
    the scroll, headstock or strap button.

Please indicate which method you used. I will give you plenty of "wiggle" room to
adjust the strap.  
"Your strap should never be an after-thought."

                                   
---Bill Bailey