SB 5.20.43

aṇḍa-madhya-gataḥ sūryo
 dyāv-ābhūmyor yad antaram
sūryāṇḍa-golayor madhye
 koṭyaḥ syuḥ pañca-viṁśatiḥ
Synonyms: 
aṇḍa-madhya-gataḥ — situated in the center of the universe; sūryaḥ — the sun globe; dyāv-ābhūmyoḥ — the two planetary systems Bhūrloka and Bhuvarloka; yat — which; antaram — in between; sūrya — of the sun; aṇḍa-golayoḥ — and the globe of the universe; madhye — in the middle; koṭyaḥ — groups of ten million; syuḥ — are; pañca-viṁśatiḥ — twenty-five.
Translation: 
The sun is situated [vertically] in the middle of the universe, in the area between Bhūrloka and Bhuvarloka, which is called antarikṣa, outer space. The distance between the sun and the circumference of the universe is twenty-five koṭi yojanas [two billion miles].
Purport: 

The word koṭi means ten million, and a yojana is eight miles. The diameter of the universe is fifty koṭi 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 koṭi yojanas (two billion miles).