In most cases a proper fitting belt is usually between 3 to 3 ½ times the same size as the waist. i.e.
A man with an average build of 5’ 9” – 5’ 10”, weighing 190 to 200 lbs with a waist size of 40” may comfortably wear a size 5 belt (120”) with a North American fit gi or a size 5.5 (126”) with a Traditional style gi.
A teenager or man with a slim build of 5’ 9” – 5’ 10”, weighing 140 lbs, with a waist size of 34” may wear a size 4 (112”) with a North American fir gi or a size 4.5 (116”) with a Traditional length jacket.
A woman with an average build of 5’ 3” – 5’ 4”, weighing 140 lbs, with a waist size of 34” may comfortably wear a size 3.5 belt (108”) with a North American gi or a size 4 belt (112”) with a Traditional style gi.