Apple Mail font size too small for father

My father just purchased a iMac with Mountain Lion OS and a 27 inch screen.

My father needs to be able to read the text of the AppleMail window.

We have increased the size of the fonts to 18 to make it easy for him to see what he is typing in the email editing window, however, none of the other display fonts in AppleMail increase.

The increase in font size is intended by Apple to provide an Accomodation to provide Equal Access for those with Sight Disabilities to the Essential Functions of use, however, this intent appears to have failed within AppleMail.

Would a different mail reader (gmail, etc) provide this font sizing ability?

Requested Remedy:

Provide the ability to increase font size for all of the AppleMail display.