You have deployed corporate email signatures, but one or a few users see a small or large font.
How To Increase the Font Size While Reading Mail in Outlook
Hold down Ctrl while turning the scroll wheel on your mouse.
In Outlook 2007, alternatively
Open the message in its own window.
Click Other Actions.
Select Zoom... from the menu.
Choose the desired zoom level.
To change this in Outlook 2010,
select New E-mail and mouse click in the body of the message text.
Click "Zoom" under Message tab.
Set the percent to 100% and click OK.
Then zoom settings will be saved and it will not change for every message.