Step 1: State its geographical spread:
The mature Harappan (Indus Valley) Civilization (c. 2600-1900 BCE) spread across a vast triangular area of roughly 1.5 million sq. km, covering present-day Pakistan and north-western India.
Step 2: Name the boundary sites:
Its outer limits are marked by Sutkagan Dor (west, Baluchistan coast), Alamgirpur (east, Uttar Pradesh), Manda (north, Jammu & Kashmir) and Daimabad (south, Maharashtra).
Step 3: Name major sites and river systems:
Major sites -- Harappa, Mohenjodaro, Dholavira, Lothal, Kalibangan, Rakhigarhi, Banawali -- developed mainly along the Indus and the now-dry Ghaggar-Hakra (Saraswati) river systems.
Step 4: Note its significance:
This made it the most extensive of the contemporary Bronze Age civilizations (larger in area than Egypt or Mesopotamia), with a remarkably uniform urban culture across this vast span.
Final Answer:
\[ \boxed{\text{Spread over ~1.5 million sq km from Sutkagan Dor to Alamgirpur and Manda to Daimabad}} \]