How to change the image size in new Media Uploader (ie use medium vs thumbnail)

Question

Is it possible to change the image size of the thumbnails in the new media uploader?

For example images are currently displayed using the "thumbnail" image crop size and I would like them to be the "medium" image crop size.

Answer 1

Supposing that I'm interpreting your Question correctly...

The thumbnails displayed in the new Media Uploader are already the medium sized ones, being constrained on the fly.

So, it's a matter of applying some CSS styling to increase their size. Maybe other style adjustments are necessary, this is just a proof of concept.

add_action( 'admin_head-post-new.php', 'style_thumbnails_wpse_81677' );
add_action( 'admin_head-post.php', 'style_thumbnails_wpse_81677' );

function style_thumbnails_wpse_81677()
{
    $thumb_width  = get_option( 'medium_size_w' );
    $thumb_height = get_option( 'medium_size_h' );

    echo "
    <style type='text/css'>
        .details.attachment, .attachement, .attachment.save-ready {
            width:{$thumb_width}px !important;
        }
        .attachment-preview, .attachment-preview .thumbnail {
            width: {$thumb_width}px !important;
            height: {$thumb_height}px !important;
        }
        .portrait .thumbnail img, .landscape .thumbnail img {
            max-width: {$thumb_width}px !important;
            max-height: {$thumb_height}px !important;
        }
    </style>
    ";
}

Result: