You can give a fixed height to the vertical stack that contains the list of collapse elements (e.g. 400px), and set its Overflow property to “Scroll” in the Design tab.
It looks like the approach you are suggesting allows me to reduce the size of the content, which doesn’t really work for my use case. I would like to be able to increase the size of the dialogue window itself, as I want to build complex UI inside of it. And this might require a larger width and height of the dialogue window.
I have found the solution and it is an obvious one.
The dialogue window size can be edited by editing the dialogue component itself, not the objects within slots.