To change size of thumbnail in Summary Block, you can do these.
#1. First, find Summary Block ID.
In my example, it is: #block-yui_3_17_2_1_1738493272859_6666
#2. Next, use this code to Custom CSS box.
If code doesn’t work, you can comment below, message or email me.
#block-yui_3_17_2_1_1738493272859_6666 .summary-thumbnail {
padding-bottom: 35% !important;
}