How to resize the element style for the text area and manipulate navigation functions , where are th

I want to resize
element.style {
height:173px;
width:956px;
}

to width:600px;

But I can't since it's hard coded by this Iframe and I can not over ride it by css
<iframe frameborder="0" src="javascript:true;" style="width: 956px; height: 173px; "/>

Could you please let me know where is this Iframe and functions concerning Navigation?
I just want to add something in the menu and I didn't find navigation functions in php files