Oh I’m still a Windows user, haven’t yet migrated over (though I do have a Nobara install I’ve played with a bit, I haven’t tried to get Apollo working on it). I stream 2560x1440 and just ignore the black bars, but I could request 2560x1600 and I think it would work just fine (I prefer the higher resolution for higher quality, rather than the native 1280x800, though I can confirm that requesting 1280x800 works when my bandwidth is limited).

That setting is handled within Moonlight, and Apollo respects that setting by default, so Apollo presents itself as a virtual display with the resolution requested by Moonlight. At least that’s my understanding.