How can I fix the height of the caption, so the "View more" buttons stay at the same height?

We don't normally fix the height, because it can create problems on the mobile view if you have very long descriptions.

Here's how to fix the height.  Please go to Admin > Themes > Template Editor.  Under "Assets", find the file called customise.css.  Add this to the very bottom:

.caption h4 {
  min-height: 40px;
}

Save.