Is there an optimal font size?

For most website projects I've been involved with, the font size for the bulk text is usually 12px - 13px, but I'm wondering if this is a good size. I remember reading an article saying that font sizes on websites should be much bigger, around 17px for bulk text to match printed font sizes.

Is there an optimal font size. When is something too big/small for the average user?

Is there any pro/cons for small/medium/large font sizes?

Update:
I would like to get your experience with actual tests user reactions etc. It's easy to this, this and that is too big or small, but I've learned that users sometime surprise and react differently from expert opinions.

Also the ability to change font sizes is a good discussion, but it's not what's asked for here. What I'm interested in knowing more about, is how users react to different font sizes, not how they should be able to change it.