Just curious why CSS is typically designed with em rather than px? how large is 1em?

1em is a relative value. If you set a default font size, then 1em is that, and 2em is the next size up.

The reason that it is good to use em, is for accessibility purposes. It's easier to use the browsers text-size editor for people who have problems with they're eyesight

Angelosanto is right...1em is a relative font size.

If your font size is set to medium, 1em is about the equivalent of 16px...however, as users resize their text, 1em will grow and shrink proportionately.

It's also more useful to use em (or percentages, as I prefer, but either is acceptable), since most browsers ignore fixed font sizes anyway.