wysihtml5-resize.js

this.editor = new wysihtml5.Editor("textarea");
this.editor.observe("load", function () {     
    var $iframe = $(this.composer.iframe);
    var $body = $(this.composer.element);
    
    $body
      .css({
        'min-height': 0,
        'line-height': '20px',
        'overflow': 'hidden',
      })
      .bind('keypress keyup keydown paste change focus blur', function(e) {
        var height = Math.min($body[0].scrollHeight, $body.height());
        // a little extra height to prevent spazzing out when creating newlines
        var extra = e.type == "blur" ? 0 : 20 ;
        $iframe.height(height + extra);
      });
});

Don't forget to set a default height on iframe body for it to work otherwise the scrollHeight is miscalculated.

awesome piece of code, thanx for sharing.