Ummmmmmm the waist size they are referring to is not your waist its where the belt will ride. I am 34 inna waist n wore a 21 inch d-ring for years.Measure yer self at the same point for a waist size as ya did for the D size. I spect it'll be close. If it is a tad loose they make a gut strap that will keep the belt from sliding down.
Ummmmmm fixed D-rings are just that fixed. floating D-rings will slide inside the body belt and allow more manunerability, Semi floating will also slide some but not much. I dont reccommend a fixed D-ring. Thats the kinda junk Ma bell uses.