I don’t understand. Here the caption box stays at the height you have defined.
I think, your problem is the following:
You want to format your text in a size, which won’t fit into the container. If your font size is too big, the container can’t be reduced under a certain limit. You can correct this – of course – within appropriate limits – by
• selecting the caption box (not the text within), and
• opening the "Spacing" panel to reduce the values for "Padding", which are set to 10 px by default.