Internal Resolution Override?

[Kurausukun]

A guy I was talking to sent me a screenshot he took in SMG2 at a ridiculously high resolution (12148x6832). I DO still have the image if you want proof. I asked him how he did it, but unfortunately he said he had forgotten... is there somewhere in Dolphin's settings to manually override the internal resolution to something that isn't one of the presets? Because that's the only way I can think of doing this.