How to Change Text Size on Your iPad, iPhone, or iPod touch
The default text size on iOS devices isn't particularly large or small, but we do have the option to change it to something more personally suitable. Whether you suffer from poor vision or just want a little change, it's worth exploring the various font sizes on your device.
On an iPad, iPhone, or iPod touch running iOS 8 or higher, simply navigate to Settings -> Display & Brightness -> Text Size. For those of you still on iOS 7, go to Settings -> General -> Text Size.
Using the slider, drag it to adjust the size of your font, which will also be applied to applications that support Dynamic Type, like Twitter.
If you increased the font size but still think it's too small, you can make it even bigger with another setting. Go to Settings -> General -> Accessibility -> Larger Text (aka Larger Type in iOS 7) and toggle on "Larger Accessibility Sizes" (or "Larger Dynamic Type" in iOS 7). Then use the slider to increase the size even more.
If you're still having a hard time seeing your now-huge font, you could even take it a step further by bolding it. Just go to either the Display & Brightness settings, or the Accessibility settings, and toggle on "Bold Text" (a restart will be needed).
This is a great tip for those who want to make things easier to read, but also for people that want to make text smaller to fit more content on the screen, like a conversation thread within Messages.