The cut width depends on the width of your seam. for a folded binding start with 6 times the width of your seam and adjust accordingly. However that formulae doesn't take into donsideration the folds of the binding both when you fold it initially and the fold to the back. Since most of my quilts have a 3/8" seam allowance I cut 2.5".
When you initially stitch the double folded binding down, then measure how much binding there is to the left of your seam line. It should be slightly more than twice the width of your seam. I found 2.5" a little wide so started cutting to 2.14" and it works better. I attach the binding to the back by hand. The seam of the binding should be consistent and a size to fill with the quilt. Practice a bit until you have found what is good for you.