From there you can tap on the Text Size menu option, and then slide the slider at the bottom to your desired text size. For bold text, you’ll need to head to Settings > Display & Brightness.

How to Make Bold Text Go Away in Emails on an iPad. You can adjust and control the bold styling of an outgoing email with native iPad applications, but avoiding bold text in incoming emails ...

New in iOS 7, you have the capability to turn on Bold Text and Increased Contrast. Note: my screenshots are done with bold text.