1) Theoretically unlimited, depending on your hardware. It must all be powers of two though.
2) Unless I'm very much mistaken, this can be done using cube mapping, you will need a shader-compatible card for it though.
3) If you mean backdrop such as 3D backdrop, these have been removed from DBP because its better implemented using a skybox or sphere. If you're talking about 2D, as far as I know once again there's not fixed limit bar that of your hardware.
AMD Athlon XP2800+
GeForce FX5200 128MB
1024MB DDR 333MHz RAM