Starhunter,

For a traditional style kilt the apron will be 1/2 of both your waist and hip. This is plus or minus a bit but not much.
This means that the aprons are tapered and wider at the hips than at the waist. Looking from the sides though the apron edges should be a vertical line.

For a modern or contemporary kilt the aprons can vary from 1/2 the waist to 1/3, and down to 7" wide for a MUG style.