I don't think you should base your upgrade choice to which edition occupies more space after installation. There should be more factors, like what you [like to] use your computer for, what you're upgrading from, if you're happy with what you have now, things like that.
In my opinion, if you have Win98, 95, or ME, then it'd be worth your while to upgrade for the increase of stability. If you're coming from Windows 2000, and you're happy with it, then you should stay with it, because there isn't much of a difference, other a broader driver support and maybe an increase of gaming performance.
There are some things you should take into consideration. Software incompatibilities, hardware incompatibilities, and non-supported hardware. If you do decide to plunge into this, make sure you back everything up. Even better, if you have Drive Image or Norton Ghost, then image your current setup, so you can go back to it with minimum fuss.
<A HREF="http://www.microsoft.com/windowsxp/whichxp.asp" target="_new">Which Edition Is Right For You?</A>
P.S. Since you wanted to know, after a Windows XP Pro fresh install, it came to about 1GB.
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