The Word plug-in formats the citations in your “Default font”

To change this, under 'Format' in Word 2003, and then 'Font', there is a button on the bottom of the window called 'Default'.

Prior to clicking the button, select the font and the size you want all text to appear, and then click the 'Default' button. It will ask you to confirm, - click 'yes' and it will make this the standard for all new text, including Zotero citations and bibliography.