Indeed, if the "sun" in the center is large like our Sun, the distance would be equal to about 1AU, or the same distance between Earth and the Sun.
What if, just for grins, that if this Nibiru is a Dyson Sphere of sorts, except it is not surrounding a natural sun, perhaps it is completely manufactured, an "artificial singularity" at the center instead of a full blown star? That could be possible, which in turn would require far less size of the sphere.
Or perhaps this Nibiru is a planet, surrounded by an artificial atmosphere or "sphere within a sphere". Maybe this outer shell, from the inside, produces the heat to keep the planet warm. (shrugs)
Its very interesting that very old ancient texts show the people of that time period had vast knowledge of the solar system, all of the planets including Nibiru, and the constellations. Many ancient monuments are alighned to certian constellations such as the pyramids at Giza, alighned with Orion's Belt. Its also very interesting that many ancient monuments are aligned with Earth's natural energy grid, or "Leigh Lines". This tells me that the ancients knew quite alot about natural energy and astrological alignments. For anceint civilizatons to have such vast knowledge of these things, yet we are told their knowledge did not go very far byond sticks and stones, I would say that the ancients were far more sophisticated than we know of.
Many think that its fables, tales and myths. Well, that could be true, if they only found a finite number of texts. But finding hundreds of thousands of ancient texts all saying the same things from different civilizations spanning several thousand years?
I find that very hard to accept that generations of ancients and the many different societies of the past, would all decide to play tricks on the future generations and make all this up by writting false texts. Especially when the evidence is all around us in these ancient monuments and their precise alignments to stars and the natural energy grid of the Earth, would indicate that the ancient societies were very serious in what they did, and in what they wrote down for recorded history.
It would be funny to see some future archeologist to find records from 1969 when we went to the Moon, and say that the task was impossible for that time period, yet we know we did go to the moon.
Almost a catch 22 isnt it!