As this is a Styles issue, moving to Styles Discussion.
This is a known thing with subSilver. The subSilverish template of phpBB 2.2 (when it comes out, no release date) will be done with relative font sizes, but there are other options for now.
You can simply edit subSilver.css and change the values to relative values. Just search for "pt" in the file and change the value (if you know how to do this). Also, I am mostly sure I remember there being an "accessible" style for 2.0, but I don't remember what it was called.
Finally, look into using another browser other than Internet Explorer. Many other browsers such as Opera
support zooming in on "fixed" text much better (I believe Firebird does the better job of the two).