About the double values for Actors, I don't think it's necessary. It won't give you a noticable effect, because pixels are still adressed using integers, not floating points. Sure, you have more variety in how much you can move each act cycle (you can then do 1.5 instead of 1 or 2), but that's just on paper: displays can't display sub-pixel movement.

@GreenHouse of course, greenfoot was designed to educatie people, and I don't think there's anything wrong with that. It's just that I don't think the Greenfoot website was built to exchange your scenarios with your teacher. And I must say, great idea! Maybe you should mail it to the Greenfoot team (support@greenfoot.org I think, not sure)

Agreed with Entity1037 on the second point: maybe schools (or someone else?) need to find another way to exchange the final projects, instead of posting them on greenfoot.org

You can also just draw all the actors that you want to magnify onto a copy of your background image, and magnify that image. That way you still add actors to the world, so collision detection still works.