I have a friend who wants to increase the text viewing size in Mail.app, she finds it too small. I have changed all the text sizes in the "Fonts and Colours" pane in the Mail.app prefs, but that doesn't affect the default text size of received messages. Also, when she sends messages, she wants to view the text as larger, without changing the actual font size (so it looks normal when others receive the emails).

Will she have to wait for proper resolution independence in 10.6? Are there system hacks you can use to do what she wants?

My parents have the same issue...

Try + (as in hold command and press the plus sign). Making it smaller is just - instead.

I think this is what you want. This works for other applications like safari as well.

It should - when you adjust the size setting for 'Message Font' it alters the display size of the text in the body and the header of all received messages, including ones that you've filed.

But as Andrew suggested: + should do the trick.

It only alters the text if the email doesn't have any formatting - if the email has customised fonts then it doesn't seem to affect things. It also doesn't affect emails as you're writing.
+ changes the text size - I want to change the viewing scale. When you are composing a message, + only affects text that is selected, if you just press + a few times then start typing, it doesn't change the font size. I don't want to change the font size anyway!

It seems as though there is no built-in way to do this... I guess Apple figure it's best to just leave it since they know that res-independence is just around the corner.