Width of embeddable markdown editor

Hello, I’m trying to use the markdown editor class from this gist:

and it’s working great so far.
However, the area you can click to focus the editor is very narrow and it’s not taking the full width of it’s parent. In fact it’s so narrow that at first I was not sure if it was working because I was trying to click on the middle and it was not doing anything, then I discovered that if I click on the first 10 pixels, it was indeed working as I was typing, the width was getting bigger etc, but this is very bad user experience.

I’ve tried attaching classes, adding minimal widths etc, and the wrappers get bigger, but nothing is affecting the area you are able to click to focus the editor.
Here is an screenshot of how tinny it is:

Probably, CSS can solve this.

How? I tried all sort of CSS settings, for different wrappers and dom elements and none of them seems to be working.
I also searched the internet for CM specific issues like that, and it is supposed that CM should fill all the space of it’s container