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 found this review 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.

Though it consume some time but I must appreciate you effort in it and it look so clean and clear to me.
Great work buddy.