This website gives you an estimate. The world size here is based of a sample of 90 worlds but each seed is different.
If your nether is greater than 1/8th the size of the overworld players traveling by nether will be brought to your world border in the overworld.
As players play on the world the size will increase. For instance a 2.5 year old SMP with a 12,000 world border has gained 1.5GB in size.