Final answer:
The four river valley civilizations varied in size based on their geographic features and river systems.
Step-by-step explanation:
The four river valley civilizations, including Mesopotamia, Egypt, China, and the Indus Valley, differed in size based on the geographic features of their respective regions. Mesopotamia, located between the Tigris and Euphrates rivers, was the largest of the four due to its extensive river system, spanning about 500 kilometers.
Egypt, situated along the Nile River, had a gentler slope and smaller floodplains, resulting in a smaller size compared to Mesopotamia. China, along the Yellow and Yangtze rivers, had a significant landmass, but its complex waterways and topography made it diverse in size, with certain regions being larger than others.
Finally, the Indus Valley civilization, located near the Indus River, was relatively smaller than the other three civilizations in terms of geographical extent.