Bib shorts sizing

Got a quick question on bib shorts for those guys & gals who wear them for cycling.  I'm about to pull the trigger on my first pair.  Performance has 20% off today only, and they have the Pearl Izumi PRO bib shorts on sale for $134.99...so that's what I'm leaning towards.  I have about 5 other Pearl Izumi shorts, all in size Medium.  But I'm unsure whether bibs will fit the same.  Should I stay with a Medium or size up?  I'm particularly concerned about the "bib" part fitting comfortably...especially since I'm a female and wouldn't want to feel like the bibs are binding or restrictive in any way.  I'm only 5'6", so I'm not sure if height is taken into consideration when sizing correctly for bibs.  Any help would be greatly appreciated!  Thanks!

Linda