Set up a new blank page in a Sandvox site
Make sure you have the option Load data from the Internet checked (under preferences) so that you'll see your changes immediately.
With the above page selected, insert an external page object
- External Page
Resize the frame to suit. By dragging, I could only get 640 px wide.
After showing the inspector,
- Click on the Metrics ruler (5th box)
- Enter the desired size.
If it changes during editing, redo it.