If you're using Internet Explorer, it's much more complex:
[ol]
[li][Alt][F4][/li]
[li][Windows-Key]R[/li]
[li]type firefox and press [Enter][/li]
[li]Use [CTRL]+ to increase font sizes[/li]
[li]Use [CTRL]- to decrease font sizes[/li]
[/ol]
I'm sorry... I couldn't resist.
Actually I looked it up and you can't change font sizes with a keyboard shortcut from IE, but you should be able to do it using [ALT]V to open the view menu, then use the arrow keys to select the text size from the menu.
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-- Ghodmode