How to always display full size image

Not a PHP solution, but below is a jQuery option.

    var images = $('.entry-content img');

    images.each(function() {
        var src = $(this).attr('src'),
            // removes the  img size off the src e.g -300x300.
            newsrc = src.replace(/-\d+[Xx]\d+\./,'.');         
        $(this).attr('src',newsrc);
    });

You would need to remove some of the classes e.g size-thumbnail