When buying bib-tights, many of the same principles apply as when purchasing bib-shorts in the summer, but fit is a more complex consideration than usual.
The additional material of a tight can pull the chamois away from where it’s meant to be, and place unwanted material where it’s not.
Pay particular attention to fit around the knee. The tight should follow the shape of the knee and be neither restrictive nor have excess material when pedaling. Many manufacturers use a pre-shaped knee panel to achieve this.
The more panels, generally the better the fit and hence how comfortable the tights will be, though that’s not a hard and fast rule. Sportful’s No-Rain bib tights have a panel-free design and rely on an anatomic construction to provide an excellent fit.
Length and sizing also varies considerably so, as ever, it’s important to try before you buy, not least because we all come in different shapes and sizes. Most local bike shops will have a selection of tights from a range of manufacturers to try on.