Don't use a very small or very big font size. Stick as close to 100% as possible. Leave it up to the visitor to adjust the size of fonts. Some visitors have a very hard time to read small fonts while others prefer smaller fonts.
Also note that Internet Explorer, when the font size is specified in pixels, doesn't allow the user to change the font size. So people with bad eye sight are not able to switch to a larger font size and hence can't read the information on your site.
It is possible to overrule all font size suggestions in Internet Explorer, but most users don't know where to find this option. If they do, the result is probably not even close to what you intended, so don't use pixels.
Also, if you specify a font face, make sure it is close to the default, or specify a good default. If you specify several alternatives make sure that the alternatives have similar properties, so visitors on some operating systems don't have to use an alternative font face that is too small to read.