Containers indeed only require you to place controls inside them, but they also shrink them to their minimum size, which is zero by default. So if you don't set one, your control will shrink to zero.
You can also play with the size hints: for example, if you want a series of controls to be laid out vertically, but have the last one fill the remaining space, you can use a VBoxContainer
and set the last control's size hint to fill, expand
.