In the Proton Mail(new window) web app, you can format text for individual emails in the composer window using the formatting ribbon.
You can also specify a default font and font size to be used for all emails you compose.
To set the default font and font size:
1. Log into your Proton Account at mail.proton.me(new window) and click Settings → All settings → Proton Mail → Messages and Composing → Composing → Composer default font/size.
2. Select your preferred default font and font size from the dropdown lists.