There are a couple different techniques you could use to enlarge the fonts in both Apple Mail and on web pages in Safari that have font sizes that are just too small to read comfortably.

TECHNIQUE 1 is to use a keyboard shortcut. Hold the COMMAND key while repeatedly pressing the '+' key on your keyboard. And on older versions of Mac OS X, you may need to hold the SHIFT key as well since . With each click of the +, you'll increase the font size. To reduce the font size if it's too big, use the '-' (minus) key instead of the plus.

NOTE: on older Macs the command key is labeled with an Apple logo and is located to the immediate left and (on most keyboards) the right of the space bar. You only need to hold one or the other.

TECHNIQUE 2 involves using the menu system. In Apple Mail, go to the 'Format' menu, 'Style' and choose to increase or decrease the size. In Safari, go to the 'View' menu and choose 'Make text Bigger' or 'Make text Smaller' note how the keyboard shortcut is presented right next to many commands that are accessible in the menus. It'll increase your efficiency by taking the extra moment to pay attention to the keyboard shortcuts.

When you change the font size for a given email message or web page, the size increase/decrease is only affective for this 'session'. In Safari, it only applies to the page you're viewing at the moment. In Mail it only applies to the message you are viewing at the moment and as I recall in versions of Mail included with Mac OS X 10.4 and earlier, the font size doesn't stick if you click away from the message and then click back to it. I'm testing on Mac OS X 10.5 and the font size adjustments to a given Mail message 'stick' even if you quit out of Mail then come back to the message.

TECHNIQUE 3 involves customizing the toolbar at the top of your Mail / Safari window. If you right-click (or hold control while clicking) on the toolbar in any Mac application that has a toolbar, you'll be presented with a contextual menu with the option to 'Customize Toolbar'. When you select 'Customize Toolbar', you'll get a drop down sheet that allows you to drag icons onto or off of the toolbar. In both Safari and Mail, you can add buttons for increasing / decreasing fonts. Once added, you're a click away from adjusting the font sizes.