Using large font in mail

My wife has retinal tares in her eyes, and until we get them fixed, I adjusted her Mail to size 36, and her iMac's screen size to a big size too. (I just tried to remember the size looking at my computer's resolution, but my computer is a year newer and can run Mountain Lion, and the selection choices look different).

This works, she can read most of her e-mail. Except the lines are closer together with less white space between them - and some e-mail is unreadable as the lines overlap. I looked in her mail preferences and cannot find a setting to change the spacing between lines.

What can I do?