Hi, you can resize by selecting the entire columns element itself instead of the individual columns inside of it.
However, for a sidebar, we generally don’t recommend using the responsive columns component, since the point of responsive columns is to make it easier for you to have layouts that switch from being horizontally stacked side-by-side to being vertically stacked, which you won’t want for the sidebar.
Instead, you can just use a plain horizontal stack with two children in it, one child for the sidebar and one child for the main content area. Let me know if that helps!
Here also are some more help on how to work with layouts and responsive design in plasmic