You are actually going to be given the option to set default fonts for a number of different scenarios, but we are going to focus on setting a default font for new messages in Outlook 2013. If you decide that you also want to set a default font for messages that you reply to, or messages that are written in plain text, you can still follow these same steps, but you will need to select the appropriate option in step 6.
Step 1: Launch Outlook 2013.
Step 2: Click the File tab at the top-left corner of the window.
Step 3: Click Options in the column on the left.
Step 4: Click Mail in the left column of the Outlook Options window.
Step 5: Click the Stationery and Fonts button at the right side of the window.
Step 6: Click the Font button under New mail messages. As noted earlier, you can come back to this screen later to change the default font for messages to which you reply or forward, or for plain text messages.
Step 7: Choose your default font, and select any of the other settings that you want to apply to it. You don’t need to worry about the Set as Default button at the bottom-left corner of the window. That will remain grayed out as you are making changes.
Step 8: Click the OK button to save your changes, then click the OK buttons on the Signature and Stationery and Outlook Options windows to return to Outlook.