If your vision is less than perfect, or you sometimes have trouble reading the small font in the Messages app, you'll appreciate this tip. There's actually a way to increase your device's font size from within the Settings app. It's a great feature that will enlarge the text within Messages, Mail, Calendar, Contacts, and Notes. I find this extremely useful for my text messages.
If you'd like to increase the font size in iOS, watch the video or follow the steps below:
Step 1: Go into the Settings app on your device and tap on General.
Step 2: Next, swipe down to the bottom and tap on Accessibility.
Step 3: Tap on the third row labeled "Large Text." Here you'll have the ability to set a custom font size. Select a size by tapping on one of the rows, then exit out of the Settings app.
The changes will take effect immediately. To preview a selected font size, open the Messages app and view an iMessage or SMS. If you'd like to select a different size, repeat the steps above.´