Updated for Snow Leopard

If you find the font sizes in Safari are too small for your aging eyes, here's a quick way to increase the font size on most web pages. First, open the Safari Preferences Pane and select the Appearance tab. Set the fonts you want to use for web pages. Note that these font choices may be overridden by style sheets used by web developers, so this step alone won't change the font size for some web pages.

Font choice on Safari Appearance preferences pane

After setting the font you want on the Appearance pane, click the Advanced tab. Put a check in the box in front of Never use font sizes smaller than and select a large font size.

Font size on Safari Advanced preferences pane

I used this trick until I got new glasses and it worked for almost all web pages.