For ease of reading on screen, use a font the has a high ex/em ratio. That is, the height of the lower case is large compared to the height of the upper case. Further, while a serif font is usually preferred for print, the screen doesn't have enough resolution to support serifs at normal font sizes. My prefs are verdana/helvetica (helvetica for Macs), arial and "trebuchet MS", in that order. For headers, a serif font is more user friendly. My preference is Georgia. cheers, gary
For body copy I tend to use (in this order): Tahoma, Lucida Grande, Lucida, Verdana, Arial, and Helvetica, with a fall-back to a generic sans-serif font if it's not available (with that combination it's highly unlikely). For headers, I use Georgia, Garamond, Times New Roman, and Times, then falling back to a generic serif font (again, in that order) if none of the above are available (again, highly unlikely). Just keep in mind that you want to use sans-serif fonts for your main text, and serif fonts for your headers. A quick and easy way to tell a serif from a non-serif font is to look for little appendages coming from the letters. If you can see them, you (most likely) have a serif font. If you see just straight lines and curves, then you (again, most likely) have a non-serif font.
You're going to have some usability problems if you use 13px for your fonts. Especially considering how many people (myself included) have poor vision (-550 in each eye, plus astigmatism in my case). At the very least, you'll want to consider 16px; ideally, 100% or 1em (so the user can resize the text). However, you'll have to design the entire layout around that font-size or else you'll run the risk of breaking the layout when the text is re-sized.
More on Dan's: Many people have vision issues or just plain preferences that dictate the default font-size for their UAs. It is good practice to base your page's font sizes on the user's preference. For testing purposes, set your own browsers to 16px, which is the 'normal' default font-size. Then test by switching font size ±2 steps. A 'good' design will survive the tests, bending, but not breaking. It may not look its best, but people in need of enlarged fonts are aware of the issues, which I'm sure Dan can vouch. cheers, gary
Yup. I just got done reading Fotolia's terms of use. I had to zoom in three times before I could read the text comfortably. I think they had it set to 10px or something - it was THAT small.