How do you change the Markdown preview font size in IntelliJ IDEA?

How do you change the Markdown preview font size in IntelliJ IDEA 2017.1? The default font size is so small it's useless.

I've tried changing:

Preferences > Editor > Colors and Fonts > Font > Primary font
Preferences > Editor > Colors and Fonts > Font > Secondary font
Preferences > Editor > Colors and Fonts > Markdown
  (this doesn't have any font or size options)
⌘->scroll wheel

I can change the font size in editor windows, but I haven't found the trick for changing the Markdown preview window font.

I also use

Preferences > Appearance & Behavior > Appearance > Override default fonts by

And even that doesn't change the font size.

I'm using

IntelliJ IDEA 2017.1
Build #IC-171.3780.95, built on March 20, 2017
JRE: 1.8.0_112-release-736-b13 x86_64
JVM: OpenJDK 64-Bit Server VM by JetBrains s.r.o
Mac OS X 10.12.3

Solution

Settings | Languages & Frameworks | Markdown | Preview font size.

Original answer for older IDE versions:

At the moment, font size for the preview can be changed only via custom CSS.

body {
    font-size: 80% !important;
}

Related request you can vote for to make it easier to change:

RUBY-19148 Markdown rendering window does not support mouse wheel font size increase/decrease