This entirely depends on your theme, sidebars are displayed and styled by the theme. If the theme uses only CSS to set the widths, you should be able to make styles for bbPress pages only that change this. If the theme is responsive with some sort of grid system, it might not be that easy to do.
Is there any css code for only css to set the widths option?
And i use jannah 2 theme.
As I said, it depends on the theme, there is no magic CSS that can do that for every theme, you should consult your theme author or hire developer to investigate and change it for you.
Is there some kind of plugin to use for modifying the width or lenght some areas of the theme?
