A family of Microsoft word processing software products for creating web, email, and print documents.
AFAIK there's no way to change what 100% is relative to screen size. However you can set your Normal.dotm template to open at a more acceptable size for you.
To do this, open Word and then choose File > Open and navigate to:
~/Library/Application Support/Microsoft/Office/User Tempates/Normal.dotm
Adjust the zoom factor as Daniel described. In addition type something on the blank page of the template and then delete what you typed. You're doing this to make Word think that a document change has been to the template that requires a save. With the page now once again blank and the zoom factor set the way you want it, choose File > Save.
Close all the way out of Word and quit the application completely. Then reopen a new fresh copy of Word and it should open to the size you set.
Hope this helps