Yeah, that happens to me once in a while. It's definitely a bug in IE, my font size gets reset once in a while. Sometimes really small, sometimes really large. It was very irritating, drove me batty for a long time.
Now I have a registry script (.reg) that I keep on the corner of my desktop which does three things:
1. Set the default location for the IE window so that it comes up fairly small.
2. Sets the IE font size back to the defaults.
3. Removes the QuickTime Task entry in the Run section of the registry (QuickTime re-inserts itself there each time I open a QuickTime movie, either from within IE or by itself).
I run this registry script out of habit before opening a new IE window (the script is on an icon with the -S parameter so it's silent), so I no longer get plagued by any of these problems. If there are other recurring bugs that can be fixed by registry scripts, I'll add them to this file, too.
The contents of the script are:
REGEDIT4
[HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Main]
"Window_Placement"=hex:2c,00,00,00,00,00,00,00,01,00,00,00,ff,ff,ff,ff,ff,ff,\
ff,ff,ff,ff,ff,ff,ff,ff,ff,ff,ae,01,00,00,f1,ff,ff,ff,5b,04,00,00,4d,02,00,\
00
[HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\International\Scripts\3]
"IEFontSize"=hex:02,00,00,00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run]
"QuickTime Task"=-