Not quite sure what the 300 is supposed to mean? Miss typo? However for iframes it would be best to use CSS :) - Ive found befor when importing youtube videos that it ignores inline things.
<style>
#myFrame { width:100%; height:100%; }
</style>
<iframe src="html_intro.asp" id="myFrame">
<p>Hi SOF</p>
</iframe>