Android
Tap on the profile icon to access Signal Settings 
> Appearance > Message font size > Select your option.
iOS
Signal matches the font size on your device. Follow steps from Apple's support page to change the font size.
Desktop
Open Signal Desktop and use the shortcut to adjust your font size:
| Mac | Windows/Linux | |
| Actual Size | ⌘ 0 | Ctrl 0 |
| Zoom In | ⌘ = | Ctrl + |
| Zoom Out | ⌘ - | Ctrl - |