Hi sashafelix,
I had a similar problem. To fix it you need to:
Find out the ID of the element on your page (you can use the Inspector on your browser to find out what that is).
Add some custom code to your Child theme, something like below, and change the width to fit your site’s sidebar width.
/*FIXED WIDGETS*/
#[id-name] {max-width: 300px;}
Hope that helps!