How should degrees be listed in an e-mail signature?

There is no formal academic convention for email signatures, although your university or institution may have formatting guidelines. You can simply include as much or as little information as you want the recipient to know.

Personally, I think your name and position are sufficient and listing every degree you have is a bit redundant (and, as others have pointed out, pretentious). People will probably infer that you have a BS and MS if you also have a PhD.

It's supposed to be an email signature, not a CV. "Dr Bob Roberts" already says that you have a PhD. The rest is redundant, because having a PhD implies that you probably have a master's degree (which nobody cares about, because you have a PhD), and almost certainly have a bachelor's degree (which nobody cares about, because you have a master's). And, hey, you probably got some qualifications in high school, too (which nobody cares about, because you have a bachelor's).

So the only reason for including all that stuff in your signature is tooting your horn. And, in an academic context, it looks crass because you're drawing attention to the obvious. "Oh, look at me, I have a PhD and a master's and a bachelor's!" Well, er, so does everybody else on the academic and research staff. That level of qualifications is implied by the fact that you work here.

If you'll allow me, i'd like to play devil's advocate. Don't list your degrees in your email signature. The signature is there to format email more like a letter. It's not the place to communicate your experience. I'd recommend setting up a personal resume website or a linkedin page to showcase your experience.

Allow your message content to communicate your command of a subject, don't rest on your letters.