Common yet flexible font size

This is probably another one that is in "CSS the Definitive Guide", but I'm experimenting with CSS techniques *now* and it will take at least one day to go out and get a book. So, to jump ahead, what would be a good way to define a common font face and size for all browsers without getting in the way of users abilities to adjust size if desired. I usually just let the cards fly and just accept the default of each browser, but in my experiments today I noticed this can very dramatically between browsers.  So I was thinking it would be cool to layout a base line that would be consistent across all the browsers, similar to what I do with margins and paddings of the HTML and Body elements.  But I sense it is probably somewhat trickier for lettering. TIA Ian