I have tried setting the Default Font Size to 12; but every time I open up Word the font size in the box shows 10.
You need to find the file normal.dot or normal.dotx. Open the file by right clicking and select open. Do not double click as that will not open the file to make changes.
Once the file is open. Change the file and save. Try to open a new document to verify the settings worked.
Assuming that you are using Word 2007.
Follow the steps to set the Default
Refer the screenshot for more info.