Yes. But it varies whether you're using iOS 6 or iOS 7.
If you're on iOS 6, in the Digg app, go to "Settings" and select "Display Settings." You can then choose between three different font sizes.
If you're on iOS 7, open the Settings app, go to "General", and select "Text Size." This preference will affect all apps that support this feature. If you want even bigger text in the articles, then in the Settings app, go to "General", then "Accessibility", and select "Larger Type."