South Dakota is around the same size as Nebraska.

Nebraska is approximately 199,099 sq km, while South Dakota is approximately 196,540 sq km, making South Dakota 98.71% the size of Nebraska. Meanwhile, the population of Nebraska is ~1.8 million people (1.0 million fewer people live in South Dakota).
Tip: This to-scale comparison of Nebraska vs. South Dakota uses the Mercator projection, which distorts the size of regions near the poles.