CSS to have columns of equal height

Custom CSS & Theme Design
1

Say there is text/callouts/etc. in one column, media in other column. I want to alter flex height in such way that my media (image) height shrinks down to match the height of the right column.
image

Here is part of the code responsible for flex items:

div[data-callout="multi-column"].callout > .callout-content > *:is(div,ul,blockquote,p) { 
    flex: 1;
    margin: 0;
}

Can someone give me guidance or link to a manual?