| Overworld | 16,000 across · 8,000 from centre |
| Nether | 2,000 across · 1,000 from centre |
| End | 8,000 across · 4,000 from centre |
The Overworld is exactly eight times the Nether, which is the whole reason for those numbers. A portal at the very edge of the Nether comes out at the very edge of the Overworld — the two never disagree, so Nether travel is usable everywhere rather than dumping you outside the world.
The End does not scale with the Nether: End coordinates are 1:1 with the Overworld, so it is sized independently.
Borders are the vanilla ones, so they render as the real blue wall — including on Bedrock — rather than being an invisible plugin fence you find out about by walking into it. The live map is bound to the same borders, so what you see on the map is exactly what you can reach.
The world can be grown later and never shrunk, which is why it starts at this size.