"Here's a tip on the iPhone that might be helpful, if you're not already aware of it," Jim Hamm says. "If, say, you're having trouble reading phone numbers on your iPhone Contacts list, here's how to get the numbers bigger and easier to read: go to Settings > General > Accessibility > turn on Zoom. Now, when you open Contacts simply use three fingers and double-tap the screen. Immediately the numbers become larger. Double tap again to reduce back to normal size. This even works on every screen on the iPhone -- at least every one I tried."
Jim continues, "Along this same vein, while you're in the Accessibility mode on the iPhone, you can set the font size in Large Text to be much larger automatically in five apps: Mail, Contacts, Calendars, Messages and Notes. You can pick the size of the font you want."