Here's how you can change the font size in Messages in iOS 6 and iOS 7. This can make messages much more readable than the default font size.
1. Open the Settings app.
2. Go to Accessibility.
3. Tap on Large Text.
4. In iOS 6 you can tap on the size font you want.
5. In iOS 7 you can use the slider to select the preferred font size.
Read more: OS X Daily