General guidelines for font and font size.
1-2 font styles, San Serif
Bold, Italics, Underline (use consistently)
If your conference or event gives different guidelines, use those.
There are no "rules," but it may help to use these dimension guidelines in your software for font size when in doubt about readability:
Title: 80 -150 pts, bolded
Body Text: 24-32pts
Captions: 18 pts