This is one that can be sorted manually with a quick bit of CSS, but tucking it away in the preferences might be a nice thing to have.
Being able to set the default --d-preview-column-min-h to something smaller would really help in layouts that involve using blank cells or columns with background colors or images, with sizes that are responsive to the height of the row rather than their own content.
I get the point of them preventing the space from collapsing, but being able to set it a bit smaller on some sites would be ace.