In case you have problem reading text in Outlook 2010 while trying to create a new message or forward a message or reply to a message then the first thing you could try checking is the zoom settings. The zoom option for message editor is available as part of Message menu. To fix the problem related Outlook 2010 email message font size, Click the Message menu in email message editor. Then navigate to Zoom section.
Clicking the Zoom icon would display the following Zoom window
In the Zoom to section, change the Zoom percentage from 75% to 100%. And then Click OK button to confirm and save the changes. This would restore the display of font size to the original value.