Changing Font Sizes in iOS

iOS goes about enlarging your font in a slightly different way although it generally appears to have larger font use in the menu's to start with, the setting only applies to fonts in your messages (and not the menu's and general usage of the phone).  To change the Font Size in iOS :

1. Open Settings
2. Choose General (on a tablet this is already the default selection)
3. Select Accessibility
4. Select Large Text

This will allow you to select a Font Size up to size 58 points.  On an iPhone this gives you about 5-6 characters wide in an SMS message so you may find it actually harder to read as words have to be split quite regularly.

You can also change the font sizes in WIndows Phone 8.  Click here for more details