change image size in media file selector of media module
The media module adds the image style 'square_crop' when it is enabled

you can alter this image style to change the display of thumbnails in the media browser (admin/config/media/image-styles/edit/square_thumbnail).

I believe the theme css also uses css to define the size, so to see the effect you will also need to make some changes in whichever theme is selected for the media browser (admin/config/media/browser):

.media-item img {
 border: 2px solid transparent;
 display: inline-block;
 margin-left: -2px;
 height: auto; /* override this in you theme */
 width: 100%; /* and this */
}