For me (Mac OS X), it's ~/Library/Application Support/Firefox/Profiles/xxxxxxxx.default/
chrome/userChrome-example.css

You could also run Gmail as it's own independent application if you're on a Mac.

http://www.hawkwings.net/2006/05/13/a-dedicated-distraction-free-browser-for-gmail/

You can also try Control + Shift + Z for larger font and Control + Shift + X to decrease

Dean, when you hit this shortcut the problem is that it will change font sizes on other pages visited with that browser window too, and also, you will always have to hit the shortcut whenever you restart Firefox (unless you switch it in the main options, but you still have the problem of the new size applying to all fonts on all pages).

You could also use Opera and use Site Preferences to just apply your user stylesheet to Gmail. Just throwing it out there :)

Long livinger? (how to compare this anyway?)
Just use this code, works at least 2.0 and up, no extensions. Needs to be placed in userContent as well:

-moz-document domain("mail.google.com") {
*{ font-size:120%; }
}

Might not be as accurate as your version, but the domain selector works without an extension.

you can install the GreaseMonkey extension for FireFox and add "gmail rl css skin"
this will apply a completely different template to your gmail, you can change everything by editing gmailrlcssskin.user.js

Use Stylish and look for other gmail style tweaks ;)
http://userstyles.org/style/search/gmail
http://userstyles.org/style/show/438

Hello,

changing "site font" is done in Opera very easy with "site preferences" as mentioned above by Ben Buchanan or with a single zoom with "View--Zoom".

greetings, Thomas