The first corner reflections would be from the corner closest to the speakers, obviously enough. So the front corners should be treated first.
Using different size traps allows you to trap different frequencies so that's not a bad idea. It would be good to determine what your problem frequencies are likely to be first, then choose a trap size for that frequency.
Using different size traps allows you to trap different frequencies so that's not a bad idea. It would be good to determine what your problem frequencies are likely to be first, then choose a trap size for that frequency.