Are you talking about the sort of thing you see in these forums when a large chunk of code is posted; to keep the size of the post (and page) under control, the size of of the box is limited, and scrollbars are added?
If that's the case then we're not talking about an iframe. CSS can be used for that: you can set the maximum size for a box, and that scrollbars should be added to the box if there is too much content to fit.
If you want to delay loading the content until it is explicitly asked for, then there are two methods.
The most powerful is to use JavaScript to make the request and populate the appropriate element (I comment on this method here). That does assume JavaScript is running on the client's end (so Google, for example, won't see the content) but like I say it's much more flexible (as any web-based application can show).
The other is (still) to use an iframe (of course), in which case it's just an ordinary request via through an ordinary link to an ordinary page (that somehow identifies in the URL which item is to be displayed, typically of the usual "&postid=271828" variety). The link has a target= attribute with the same value as the name= attribute of the iframe that the requested page is to be displayed in.
