If you want to change the default font size (text size) in the Gmail app without changing any other font sizes. This one is easy, but it's buried in some menus. Launch the Gmail app, then while in Gmail do the following:
1. Tap Menu
2. Tap More
3. Tap Settings
4. Tap Message Text Size
Choose the size you want and you're all set.
1. Tap Menu
2. Tap More
3. Tap Settings
4. Tap Message Text Size
Choose the size you want and you're all set.