This civilization began around 2500 b. c. and was the largest of the worlds first civilization

That would be Egypt! While Upper and Lower Egypt had been unified prior to ~2500 BCE, it was around that time that Djoser initiated what has come to be known as the Old Kingdom. At its height, Old Kingdom Egypt stretched from the Nile River Delta in the north all the way to the First Cataract of the Nile (the modern-day location of the Aswan Dam). 

Geography