You can change the site easily. Did you look at the text inside "Copy Embed Code" ? It looks like this: ...type="application/x-shockwave-flash" width="380" height="349" data="http://getembedplus.com/embedplus.swf">...There's width and height specification like standard YouTube Embeds. Just change it. Also, the wizard allows you to set the size to a new embed during step 2.
Really, setting is during step 2, I tried only site on my screenshot.