Small video overlay smooth transition to full screen?

[meshuken]

This must be simple, don't know why I cant figure it out:

Video track is a newsanchor sitting at a desk. Overlay track is the reporter in the field. As the anchor is sitting at the desk, the overlay track appears from bottom of screen and moves to the top (simple enough) over his right shoulder.

As the repoter begins to speak, I want the small "overlay track" to smoothly increase in size until it fills the entire screen.

I can't figure out how to do this!

Thanks,

M