If viewing HTML content use Ctrl+Page Up to increase and Ctrl+Page Down to decrease.

If viewing plain text, while a reader window is open click the Reader pull-down menu the select Font.  You then set the default font and size for the reader window when viewing plain text.  Notice that the Reader pull-down menu is only visible when a reader window is open.

My messages are really small print. I can change the font size for the headings but no luck with the main message. Helen Allen

[quote user="Helen Allen"]My messages are really small print. I can change the font size for the headings but no luck with the main message. Helen Allen[/quote]

Assuming you're printing plain text messages goto Tools => Options => Message reader => Use alternate font for printing messages in the middle of the page.

Thanks for the answer but what I want to do is change the size for reading messages.not printing.