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The advantage of fixed font sizes is that everyone sees (almost) the same thing.
The disadvantage of fixed font sizes is that people with less-than-perfect vision cannot adjust the size to suit their personal needs.
You might want to check your Netscape installation verrsus your IE installation, and compare the default font size - I believe both used to ship with it set to 12 or 13 points, which is not tiny. This is the number that you are adding +1, -2, etc. to when you use relative font sizes. If the browsers are set to different "base" sizes, then of course the on-screen size will look different.
There has been a lot of discussion of font sizing in a CSS context, although some of it predates the CSS forum here at WebmasterWorld. You might turn up some useful ideas searching on "px" and "em"
Jim
There is also no agreement on how much bigger x-small is than xx-small, etc. The spec advises a step size of 1.5 (CSS1) or 1.2 (CSS2) but browsers are free to use any value they like.
These things will make a difference for anyone who varies font sizes using the names xx-small, x-small ... xx-large.
Best bet is to use percentages. That should be consistent across all browsers (NN 4.x excepted as usual, I suspect).