I have a Macbook Pro, and I noticed upon upgrading to OS X Leopard, that the default message size for Mail.app messages was too small to read. Sure, you can add a button to your toolbar to increase font size on a per-message basis, but this gets old after the 1000th time. Some Terminal magic will fix the problem:
To increase the minimum font size for HTML messages, do the following:
- Open Terminal.app. Spotlight search on "Terminal", press Enter on the app.
- Type the following at the Terminal prompt and press Enter:
defaults write com.apple.mail MinimumHTMLFontSize 13
Now after a restart your HTML messages will have a base font size of 13 points, which is much easier to read, and you can set the font size to a point size you are comfortable with.