Opinion based? Not really, because it's been actual hardware, not some fantasy console.
If you want the experience authentic, there's only one valid viewport size: 256x224 pixels.
As for sprites, pretty much anything is allowed. The original games mostly focused on 8x8, 16x16, or 32x32 sprites, but a few games also created bigger combinations.
There were also hardware limits on how much could be shown/combined on screen at a time, but I'd say most probably wouldn't notice any violations there.