I think it'd be best to have a spritesheet, in PNG format. The size can be around 2-3Mb-ish I suppose,
If someone want this style I don't think they can bargain too much with image resolution :0
I'd say inspire yourself of Kenney work and do a grid with blocks of floors and some objects.
Since it's a free assets you can start small and if reasonable requests came in, you can try and add more later 0:
I don't know too much about that side, so it's wild guesses ʕ•ᴥ•ʔ