To use custom sized-thumbnails, you will first need to select the right template.
Edit your shopping page, then under "Layout" at the top right, choose "Shopping (UL CSS)".
Next, under Design, click on "Colours and Background Images".
At the very bottom of the page, click "NOTE: Image preset size management has moved here...".
From the dropdown, select "Category UL List Thumb". then choose a size. Click "Add Image Rule".
Note: This page layout is intended for advanced users familiar with CSS. You may need to add some CSS to make the cart look exactly as you want it to.