It is not possible in Blue/Green screen to adjust the size of multiple videos to insert into one clip. However, with some fiddling it can be done in iMovie 10. iMovie 6 does not have a green screen feature.
Start out with your clip that shows the three blue parts that you want to make into video display screens:
Use the Crop to Fill feature to zoom one of the blue screens, and take a screen shot of it, leaving some margin around it, like this:
Create a new project and import the blue screen shot back into the project and overlay it as a blue screen over your video like this:
Export out the above and import it back in. Then use the Picture in Picture feature to overlay it onto your original three blue part clip like this:
Repeat the procedure for each screen. With the vertical shots you may need to crop them in the media browser (not the timeline) to eliminate black bars on each side.
EDITED
-- Rich