Wow, I have a Spanish-layout keyboard, but this is what I do to change the font of web page. To make it larger, I simultaneously press Ctrl and the key that sits just to the left of my Enter key (on the top row, you know the Enter button is two rows "tall"). On my keyboard, this key contains the +, the * and the ]. Doing this refreshes the page in a font one point bigger. To make it smaller, I press Ctrl and the key that sits just to the left of that one, which for me contains the grave and circumflex accents, and [.

I hope this works the same for you. If not, there must be a different method such as the one Jackie describes, or even a more straightforward key shortcut.