Settings for Scroll Child layout

Hi
What settings do I need to change in a box/Child layout set to “Scroll” to actually make the items scroll? At the moment, when I add more elements, they just shrink to fit the screen. What I am trying to do is only show 3 elements on the page and then allow the user to scroll to see the other elements.

i.e. I am trying to replicate what was asked here but using a manually created block in the CMS:

See attached what is happening to my block at the moment: