Chashana, when you say you can't change it, it's not clear what you mean. The font size in a new document is based on what is set for the Normal style in the Normal template. Open the Styles pane from the Styles section in the Home tab and right-click on Normal, or right-click on the Normal styles in the Quick Styles display in the Ribbon. In either case, choose Modify, then change the fort size to what you wish, and then select the "New documents based on this template" choice at the bottom of the dialogue. When you close or save this document, you may get a prompt asking if you wish to save changes to the Normal template; say yes.

Best,
Ulodesk