In layout mode:
Select the object (click on the page you added)
Grab one of the little black squares in the corner
Drag it to the size you want.
It works just like it would in PPoint, Word or other graphic paste operation. The main exception I see is that your cursor won't turn into an 'Drag Arrow'...it'll just stay a lowly cursor.
Use the cool grey lines in layout mode as references to where the page edges are.