Optimum text size?

I am wondering if there is an optimum font size or browser setting that is usually designed to? I am trying to make my site as compatable as possible for all settings, so even if a user uses larger or smaller font sizes in their browser the page is clearly legable. But even so, i have to design for the 'average' user, as you do for resolution and so forth. So i guess my question is, which is the 'average' setting of a users browser to display font size? Currently im going with 'medium' as thats IE default, im not sure about FF (which i normally design to, but im at work ) but i may be wrong in assuming this.

Thanks in advance.

Really depends, but using em's is never a bad approach. While IE limits the availability of font sizes the other browsers do not and FF itself has far more sizing options available to the user using a key stroke or like IE the scroll wheel on a mouse, however in Firefox 3 the zoom features zooms the entire page and not just type sizes.

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Well the default size on FF3 is 16 points, but obviously you can alter it to taste through CSS. However, you can't second guess a persons preference so for general text enclosed in a <p> tag, I'd go for a 10 point size.

10 point may generally be a bit small and using fixed values such as pt and px are not necessarily the best approach. 0.85, 0.9em etc are generally safe best depending on your layout scheme etc.

Just keep in mind css is overridden easily.
User set styles override everything except items with "!important", inline overrides linked etc...

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Yes, you are completely correct there. I don't use em's very often so I couldn't comment on the best approach with them (I'm going to experiment now). I just try to set a happy medium size for all and if people want to adjust, override etc, then they are welcome to do so. However, does the average user (unless sight impaired) change their default setting often? I'm not being awkward or argumentative, just curious (as I don't)!

Generally I think most users don't modify the font sizes, the advantage to using say em over px is if you use a fixed value IE won't resize it at all. And em based fonts are proportionate to the container so they scale well if needed.

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Ah, I see. I'll remember that in future. Thanks very much for the info!