Different widget widths on different theme areas?

Hi,

On my site I have one width size of the sidebar on single posts/pages and another width on arcvive/categories/homepage.

How can I have the widgets fit width of the sidebar they are into?
If I have a class with certain width, it won’t be appropriate to both widths needed.
I thought to remove the width from the css, or tried even width:auto, but then there are some widgets that getting different widhts, and they are not all with the same width.
How can that be solved?

Thanks