That isn't a Vista ...
problem, it is a productivity software problem and could best be answered in this forum:
When you created your post there was a message right at the top of the window that reads:
"Create a new discussion:
Note: If you're asking for technical help, please be sure to include all your system info, including operating system, model number, and any other specifics related to the problem. Also please exercise your best judgment when posting in the forums -- revealing personal information such as your e-mail address, telephone number, and address is not recommended."
While you posted in the Vista Forum giving us a reason to ASSUME your OS is Vista and you mention Excel there is nothing in the post to let anyone know what version of Excel you are using and what Word Processor you are using (version and publisher). Different versions of the MS apps have different steps to solutions for a given problem and if the Word Processor is from a different version or different publisher that too makes a difference.
Having said all of that to guide you to a solution I will suggest that you take a look at the formatting of the fields being populated from the Excel file. When setting up the mail merge you create a main document then you set it up then you select the fields then, if using MS Word you have an option to EDIT that main document at which time you select the font of your preference over the default 12 point Times New Roman