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You can do this in IE by setting your font to a 'px' size value, but the more important question is 'Why would you want to?'. If users that would need to up the font size can't then they won't use your site, so you're not losing anyone if it just breaks. However, there's a lot to gain if your users can up the font size and have the site work for them, and there's nearly always a way of getting your design to work with changes in font size.
So can we help you to make your site cope with font size changes?