Hi Matt,
If you not opening a document via a separate template, then the change may
have to occur in your Normal.dot template, which is the template Word uses
by default. The Normal.dot template uses default styles called Header and
Footer for anything that you first type into a header or footer
respectively. You could go into the menu Format/Styles and change the Header
and Footer styles. Anyone else opening your document may have those styles
set differently. You shouldn't try to change anyone else's Normal.dot
template. Check first to see what style that line is using by putting the
cursor there and looking up in the toolbar area in Word for the style name
textbox. If it says "normal" then just select the line and change the size.
If it says some other name, then either change that lines style
characteristics or change the style to a different style.
If you are using a template, it may have been set up that way in the
template. You can change a template by opening it up and editing the footer
line text (style?) to the size you want. The template file name will have an
extension of .dot at the end. Any changes you make will take affect on any
new documents opened with that document from now on.

For more info on styles:
http://www.shaunakelly.com/word/styles/TipsOnStyles.html
http://www.addbalance.com/usersguide/styles.htm

For more info on templates:
http://word.mvps.org/faqs/customization/WhatTemplatesStore.htm
http://www.shaunakelly.com/word/templaterelations/index.html