Recently, I've noticed that the font has changed in Gmail to being really tiny and difficult to read. I googled a few suggestions and here is what I got:

1. Change the gmail theme back to default (didn't help)

2. Change the zoom of the page back to 100% by pressing CTRL and 0 together (didn't help, was already on 100%)

3. Change the text size to medium using Page > Text Size (nope, already set to medium)

4. change the default webfont in the accessiblity options - tools > internet options > general (tab) > under Accessiblity click the button 'fonts' (nope, didnt work either)

I inspected the page elements using the new IE9 F12 feature and saw that the font size of the editor is set at '75%' in the CSS. Since the font sizes look fine in Firefox 4 and the latest version of Chrome, I can only speculate that it is the inheritance of font sizes in the style sheets information that is the issue here.

Incidentally, the same problem applies to the text on the page when you try to set up a new google account and I've noticed the tiny text problem appearing in a few places on google sites.

The only fix that I eventually found was to enable Compatibility View buy clicking on 'Tools > Compatibility View'. You can add gmail to the list of compatibility view websites by clicking on Tools > Compatibility View Settings and there you can add gmail to that list

Ironically, when you fix the font size by using compatibility view, you are then presented with a big red box containing the following warning on the top of your gmail list: