Bib Tights/Shorts sizing

Hi - I'm finding that my existing tights have a tendency to get pulled down during rides and am beginning to consider a cheap pair of bib tights to prevent this (the likes of FDX etc). However, I'm not sure what sizing would work best as I'm tallish but pretty slim (5' 11" and around 62kg) - for shorts I'm often a small, but with tights suspect this may not work due to my height. Can anyone here of similar dimensions advise on any providers/sizings that work well at the budget end of things? Many thanks!!