Go to the BP_ContainerWindow widget and then in the Designer view select the size box, then in the details panel you can set the windows max and min size.

This can be used to control both the width and height. Max size is used when the content in the window exceeds the min size value. So if you have a container item with alot of cells in the width the window can be larger in the width to allow for the extra slots, if there are to many slots in the width to fit in the max size as well then a horizontal scroll bar will be visible.