try this before you try anything more complicated
My simple fix :
1. open a new mail message.
2. press ctrl and mouse scroll until you find a size that best for you.
3. Close the message, and your preferred size will be saved.
I've never experienced this, but doing a little poking around I found this article:
http://office.microsoft.com/en-us/help/HA010547911033.aspx
Within
it, it says "If you want all new documents that you create to have a
specific font, a graphic such as a logo, or specific ruler and tab
settings, you can modify the Normal template. "
This would hold true even if you just opened up Word and tried to amend a new document.
I
would start there, and if the issue is the normal.dot, try renaming the
original and copying and pasting a known good template from perhaps
your computer (if they are same version).
You can find the default template by going to TOOLS -> Options -> File Locations ->user templates.
Good luck!