The following comes straight from the Slimbox extended website.

Slimbox is Lightbox clone by Christophe Beyls. This extended version has additional features as follows.

• Support to show external content by iframe. You can assign url not only for image but also for general html document.
• Support to set content size. You can add width/height parameter in rev attribute of the anchor url.
  e.g.) rev="width=500, height=300" (For image content, you can also use percent value.)

Demo (click to view)

iframe content

Google | Yahoo! | local iframe content

width/height parameter

Image1 (Original size) | Image2 (50% size) | Iframe (pixel value only)