PyCharm how to adjust gutter width?

The other day I mis-clicked on and mis-dragged something probably and then the left "side bar" of the editor window got wider than normal.

Since this happened by mis-clicking and dragging, I don't know how to reset it back to the normal width. It simply doesn't respond when I try to click and drag the borderline. Didn't find anything related in Settings either.

It's not possible to change the gutter width at the moment. It adjusts itself automatically when needed (e.g. there is a gutter icon for override method and a breakpoint on the same line). Looking at your screenshot, the width is actually standard. It needs some extra space to allow you setting breakpoints without accidentally clicking the line number. You could submit a feature request/usability problem to https://youtrack.jetbrains.com/issues/IDEA, but to me, it seems fine.

But since you say it was different the other day and then something happened and it became wider, you could try starting PyCharm with default settings by renaming/removing configuration folder. Here is the article on how to find it.