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:
