If you're that uncertain about the length, buy a 60" bamboo stick or another cheap stick of that length that you can cut and discard once you're done.
Apply a string or piece of duct tape on the middle of said stick-thingy so that you can wear it across your back like the Hayabusa does. Adjust each side of the stick-thingy until you have a length that you feel comfortable with. Preferrably it would be done in small steps, but if you screw up, your stick-thingy should be cheap enough not to ruin your economy if you have to buy a second one.
Once you're satisfied with the length, measure it. There's the length you're looking for. Bottom line is that if it looks too long for you, it is too long.