Probably Windows Mail, unless you've downloaded Windows Live Mail. Should be the same for both though.
The fonts used in the message lists and for the folders are the same as for the Icons on the desktop, so if you increase those, you'll increase the font size in Windows Mail.
Right click the Desktop, click Personalize
Click on Theme, and save your current Theme so you can restore settings if needed.
Click Window Color and Appearance
Click the Advanced... button
Select Icon in the Drop Down box, and adjust the font and font size.
Click OK, then click Apply and see how it looks.
The Compose window fonts can be set here
(Ctrl+Mouse Wheel does work here):
Click Tools | Options
Vlick the Compose tab
Click the Font Settings... buttons, and adjust as desired.
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