Fit varies between brands as does how stretchy the bib straps are
Some brands (like assos) also do a long and short leg version of the same size. Also other brands (like skins) have a size chart that goes by height and weight.
Generally though i go by height, if you are average (or either a bit skinny or a bit overweight) then its the easiest way to measure up as bibs are more forgiving around the waist than other sorts of shorts.