how big is a nether portal?
This is right. The world's usable size is 60,000,000 x 60,000,000, and the nether is exactly the same size. But due to the 1:8 ratio between the two worlds, portals in the nether only work within a square of 7,500,000 x 7,500,000. HUGE. You don't even need to know how big is the nether to be able to build in it.