I'd say single-screen sizes of old computers would be considered small-medium. GBC at 160×144 would be a good example of something small. Medium size for me would be GBA at 240×160, and 80s/90s desktops. ZX Spectrum 256×192 up to DOS/Amiga 320×240. Anything much bigger than that, so backgrounds that span multiple screens, and I'd say it's large.
Time estimates really depend. Let me give you some examples for myself (I feel I work quite slow).
You could do a chicken-scratch painting or concept art in 2h (a style that's often not technically pixelart anymore). 300×160:
Same with using things like
HD index painting that does dithering and anti-aliasing automatically, you can do the same size image in 2-3h. 340×192:
This is more a lo-res digital painting, than pixel art. But Dan Fessler has better examples that are more pixel art-ish.
A simple, but manually (semi-)cleaned up background goes to 10h. 320×160:
But what really matters is how many details there are (what I call 'density' of the image). Here's a bigger scene that took in the ballpark of 50h. I optimize speed of dithering using my own
technique with layers. 512×192:
And finally, this is not really a background anymore, but a good example of extreme density (while also being a big canvas to begin with). Took 200h. 540×382:
So as you can see, the range is all over the place and due to density of details and amount of cleanup, has less to do with canvas size than subject/style. And certain optimization techniques (plus your experience) again influence the time.
