Hi,
Assuming that you are using version iMovie 10.x.x, you could try the Paste Adjustments feature from the Edit menu.
Here's a link that will show you how to use the Paste Adjustments feature:
http://help.apple.com/imovie/mac/10.1/#/mov58103651f
When you get into paste adjustments, the video overlay settings is described as follows:
"Video Overlay Settings: Applies only fade and opacity adjustments to cutaway clips, split-screen clips, picture-in-picture clips, and green-screen and blue-screen clips."
It says it applies only to fade and opacity adjustments, but try it and see if it will replicate the size and position of your PIP overlay.
Or you could try the crop adjustment in the paste adjustments option, and see if that works.
-- Rich