How to make image uploads default to full size?

It doesn’t look like that filter exists in WordPress core. That being said, I have the solution for you. That select uses an option to set the default size:

get_option( 'image_default_size', 'medium' );

So all you have to do is update that option:

update_option( 'image_default_size', 'full' );

For reference: media.php line 1296