There are font size increase/decrease buttons in the lower left corner of Webmail, and these also control the font size in the Compose window in Runbox 5 Basic mode.
Now we have fixed a bug so it also works in Runbox 5 Enhanced mode where the Compose screen opens in a new window.
It doesn’t yet work when writing HTML messages since the HTML editor (TinyMCE) is a separate program and isn’t entirely integrated with Runbox.
For more information about the different versions of Runbox, please see the Webmail Help page.