Here's the options suggested by Apple to increase text size:
Change the font size
1.In iOS 8, go to Settings > Display and Brightness > Text Size.
In iOS 7, go to Settings > General > Text Size.
2.Drag the slider to make the font bigger or smaller.
You'll see this font size in apps that support Dynamic Type for your version of iOS.
Make the font even bigger
1.Go to Settings > General > Accessibility > Larger Text (or Larger Type in iOS 7).
2.Turn on Larger Accessibility Sizes (or Larger Dynamic Type in iOS 7) for bigger font options.
3.Drag the slider to select the font size you want.