This is something you can adjust using the Text Size option in Settings. In iOS 15, add Text Size to your Control Center in the Settings app. Then open your Control Center and tap the Text Size widget while the Mail app is open. Select Mail Only and use the text size slider to make the display font bigger in the Mail app. If you're using an earlier iOS, go into Settings>Accessibility>Display & Text Size and select Larger Text. Adjust the text size using the slider. This will apply your setting to all apps that support dynamic text, including Mail.