I made this belt for my daughter back in December. This was my first time. So I measured her waist and knew I wanted it a few inches longer to account for her growing (she's 3 yrs old) and I used a brass buckle from a belt from the Goodwill ($1).
I used the rule of 1 foot for every inch. I tucked the working ends into the belt for a clean look and made a belt loop so the end doesn't protrude through her shirts.
So to recap
1. Measure the waist and add a few inches.
2. Choose your knot.
3. Use the rule 1 foot for every inch (works well for cobra knot a.k.a Solomon Bar).