I'm looking for a way to set the width of the scrollbar in may class which inherits from wxScrolledWindow. I wouldn't mind editing the framework source code (3.1.0). I've looked there but I could not find the solution for my problem since the wxScrolledWindow and its base classes don't seem to use the wxScrollBar class.
You can't change the width (or any other aspect of the appearance) of the standard scroll bars, used for scrolling a window -- this is part of the native look-and-feel you get when using wxWidgets.
If you absolutely need to do something different (why?), you have no choice but to write your own scrolled window class using a plain window without scrollbars and one or two standalone wxScrollBars for scrolling it. Needless to say, this is going to be complicated and won't work as well as native scrolling, so you shouldn't do it unless you really can't live without this.