Make UI Children Fit the size of their parent

So I've been following the Unity Shop UI tutorial, which is effectively a tutorial on making a Scrollable list of elements, but have missed something along the way.

In the tutorial they make and add a prefab "SampleButton" to the list that contains an Icon next to two text elements. They then use a heirarchy as below to make the UI:

ScrollView - ScrollRect
ViewPort - Mask + Image
Content - RectTransform + VerticalLayoutGroup + ContentSizeFitter
Buttons - Image + Text + Text + LayoutElement (each children of the previous, sorry I don;t know how to indent in this markup).

The Content VerticalLayoutGroup Forces children to expand width wise but not height, and the contentSizeFitter sets vertical fit to min size. The buttons LayoutElement defined min height of 100.

Everything looks good and works (scrolling etc) with a sample button/s that have the default text and icon, but when I add new text or an icon the buttons become HUGE, with no respect to their parent size.

How do I make the parent constrain the width of these buttons (and possibly the height)? In the Tutorials when icons and such are added they simply display nicely at the right size, so I suspect I've missed something in the layout setup of the buttons. Any advice appreciated! I'm only asking because my forum searching seems to only return the opposite question (how to make parents fit their children). Beginner here so advice welcome on question format as well.

Semi relevant: is there a way to set a maximum size for elements?