You can change the font size of your emails in MobiMail.
Here's how:
Use the + and - buttons at the bottom right of the window, next to the zoom percentage.
Alternatively, use the keyboard shortcut Ctrl + mouse wheel up to zoom in and Ctrl + mouse wheel down to zoom out. The cursor must be hovering over the email for the shortcut to work.
Note: At the moment, it's only possible to change the font size of your emails and not the entire interface.