The border's on opfactions aren't correct as given by cyp. The world size in the overworld is 60,000 by 60,000 and the nether is 20,000 by 20,000. However the map is a circle so the corners of the world aren't accessible.
I'd try to work out the radius or diameter of the world but nahh EDIT Apparently it's a diameter of 67703 in the overworld and 22568 in the nether??
I know about OPFactions, just not this specific question. I see Mega on OPfac's alot so his answer could probably help.
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