I'll see your large text size and raise you larger text size.
As previously discussed, iOS 7 features Dynamic Text, which lets you across the entire device. The slider for Dynamic Text can be found via this path:
Settings > General > Text Size
If you have poor vision and find that the largest text size via the above setting still isn't legible, there is a way to increase its size even further. Head back to the General page in Settings and tap on Accessibility, which is directly below Text Size.
Next, tap Larger Type.
At the top of the next screen, tap the switch to turn on Larger Dynamic Type. When enabled, the slider below doubles in width, giving you twice the range of the slider on the Text Size menu.
As long as an app supports Dynamic Text, it will display text at your preferred size.
For more on Apple's latest mobile operating system, I will refer you to CNET How To's.