function onElementHeightChange(elm, callback){

    var lastHeight = elm.clientHeight, newHeight;

    (function run(){

        newHeight = elm.clientHeight;

        if( lastHeight != newHeight )

            callback();

        lastHeight = newHeight;

        if( elm.onElementHeightChangeTimer )

            clearTimeout(elm.onElementHeightChangeTimer);

        elm.onElementHeightChangeTimer = setTimeout(run, 200);

    })();

}

onElementHeightChange(document.body, function(){

    alert('Body height changed');

});