This article explains how to change Gmail's default font options. It includes information on making on-the-fly changes using the formatting bar at the bottom of the composition window.

Gmail's default text options help you get your message across. If you want to spice up your communications, change the text options to something that better reflects your personality. Do this for an individual message, or change the defaults so Gmail uses your preferences every time.

Use Gmail's general settings to change the default text that Gmail uses when you compose emails. When you change the settings, you choose the font, size, and color of the text. After you save your changes, these styles will be used for every email you send unless you change them again.

Open Gmail. In the upper right corner, select the Settings icon (gear).