How to make TinyMCE text bigger?

How to make the text inside TinyMCE bigger? I know that I can make the text bigger in my Firefox, but still, I have to teach my editors to do that

You can simply create a stylesheet for tiny and refer to it MODx -> System configuration -> Interface & Features -> Path to CSS file

Thanks. But the text has become giant now — almost as my <h2>

My CSS files contains only this:

body {
font-family: "Trebuchet MS", Arial, Georgia, serif;
font-size: 12px;
}

Even if I delete this, or set "font-size" to something different, the size of the font in TinyMCE remains the same.

test like it
body {
font-family: Trebuchet MS, Arial, Georgia, serif;
font-size: 12px;
}

Thank you! That worked!