How can I zoom in with Mac Mail on a message I am composing?
This isn't exactly what you want but may be a viable workaround if no one comes up with a way just to zoom the text in the composition window. In the Accessibility Preference Pane (on Yosemite, don't know if it's the same in El Capitan) there's a Zoom subpane. There you can set up various ways to zoom the whole screen. I have "Use scroll gesture with modifier keys to zoom" checked and Command as the modifier, so I can hold down the command key and scroll and the screen zooms in and out.