Quote: "Portals of different sizes could work though, even if a multiplying portal wouldn't
It could be that for some areas you need to be very small, and for some you need to be large so you have to go through the right portals to get to the correct size"
Yeah.
You might need to be large to jump over a gap or something. The mirror inversion portal would be useful if you had a keycard or a key to use in a lock.
Quote: "The way I see it, different sized portals are physically impossible, because they are an intersection. They have to be the same size. But the virtual world is different, so there it would be possible "
Like Diggsey says, I don't think there's anything wrong with having differently sized portals from a game physics perspective. Of course in real like they'd violate laws of nature:for example you can't change the size of atoms, but this is portals we're talking about.
I think if someone wanted to do a game with some originalish Portal gimmicks, Alice in PortalLand with mirror (Looking Glass) portals and size changing (Eat Me and Drink Me) portals might be a good idea.
Quote: "As to the multiplying portal... I have no idea what would come out if you reversed it"
Yeah dude I'm clueless about that one as well.