SB 5.20.43
anda-madhya-gatah suryo
dyav-abhumyor yad antaram
suryanda-golayor madhye
kotyah syuh pañca-vimsatih
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The sun is situated [vertically] in the middle of the universe, in the area between Bhurloka and Bhuvarloka, which is called antariksa, outer space. The distance between the sun and the circumference of the universe is twenty-five koti yojanas [two billion miles].
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The word koti means ten million, and a yojana is eight miles. The diameter of the universe is fifty koti yojanas (four billion miles). Therefore, since the sun is in the middle of the universe, the distance between the sun and the edge of the universe is calculated to be twenty-five koti yojanas (two billion miles).