Trying to figure out the CSS that will limit the size of/number of items in a dropdown menu for an archive block

The first image is the dropdown closed, and the second is the dropdown open. 

That second one is "fine" but obviously it would be better if it just displayed, say, 10 items and allowed the user to scroll down to see the rest.

I can change the height of the dropdown using CSS currently but that only keeps it from enlarging the section (which makes it worse), but it still displays all of the items.

Any ideas on how to tell it to just display a few? 

site is https://www.discologist.com/livemusicarchive