Hi,

1/ Please add more this code to Customize > Custom CSS to do that:

.penci-home-popular-post .item-related > a{ margin-bottom: 0; }
.penci-home-popular-post .item-related h3{ display: none; }

2/ The image size from Homepage Popular Posts will follow the images size you selected via Customize > General Options > Featured Images Type > select change to Square or Vertical. There is no any option for change the size for that place only to the size you want.

Best Regards,

PenciDesign