Hi,
You can't do that in CSS.
You'll have to edit the view file which displays the cart in order to change the size of the images there, via the menu Display>Views.
If it's the cart module where you want to change the images, then it's the file "cart" of the view "product" that you want to edit.
You'll find that code:
$img = $this->imageHelper->getThumbnail(@$product->images[0]->file_path, array('width' => $width, 'height' => $height), $image_options);
where you can change $width and $height by the number of pixels that you want.
If it's the cart display on the checkout, then it's the file "show_block_cart" that you want to edit.
There, the code to change is:
'width' => $thumbnail_x,
'height' => $thumbnail_y