How do I set the maximum line length in PyCharm?

You can even set a separate right margin for HTML. Under the specified path:

File >> Settings >> Editor >> Code Style >> HTML >> Other Tab >> Right margin (columns)

This is very useful because generally HTML and JS may be usually long in one line than Python. :)

For PyCharm 2018.1 on Mac:

Preferences (⌘+,), then Editor -> Code Style:

For PyCharm 2018.3 on Windows:

File -> Settings (Ctrl+Alt+S), then Editor -> Code Style:

To follow PEP-8 set Hard wrap at to 80.

For PyCharm 4

File >> Settings >> Editor >> Code Style: Right margin (columns)

suggestion: Take a look at other options in that tab, they're very helpful

Here is screenshot of my Pycharm. Required settings is in following path: File -> Settings -> Editor -> Code Style -> General: Right margin (columns)