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If you have ever found yourself squinting whilst trying to read text in Notes or Mail on the iPad or iPhone, this quick tip is just for you.
To change the font size, open the Settings application, select General, Accessibility, then Large Text. On the Large Text screen you finally will be able to select your desired font size from the ample list provided. The size you select will be made the default font size in Mail and Notes. Click the Home button to exit, or double-click to switch to another application.
NOTE: For those interested in changing the font Notes itself uses, read this post.
UPDATE 10/18/2011: iOS 5 now enables one to use bold, italics, and underlining in email messages. Read here.