Great question. I really doubt it would be able to support any resolution. I'm sure that behind the scenes it scales it down to somewhere around 1 mp before processing even if they decide to upscale and return it back at the original resolution.
I don't know. All the testing I've done has output the standard 1024x1024 that all these models are set to output. You might be able to alter the output params on the API or AI Studio.
Most of my photos these days are 48MP and I don't want to lose a ton of resolution just to edit them.