How to increase font size on incoming calls
 

How do I increase the letter and number size on incoming calls on both the outside display and on the main screen?

You can't.

well i have no idea if any such phone setting exists but you might search internet for your phone themes which may help you in this way :)

How do I at least get the incoming calls to light up on the outside screen?

Did you read the above post? You can't without finding another theme.