Behind the speakers first.
Yes you can use different sizes for the front and back.
Yes you can use different sizes for the front and back.
bass traps..which wall first
when using bass traps (any kind for this question/answer) which wall should be the priority, the rear corners behind the speaker, to the corners behind the listener? Can 2 different ones be used "safely" for example can you use 4' tall traps behind speakers, and 2' behind listener and vice versa?