South Dakota is around the same size as Senegal.

Senegal is approximately 196,722 sq km, while South Dakota is approximately 196,540 sq km, making South Dakota 99.91% the size of Senegal. Meanwhile, the population of Senegal is ~17.9 million people (17.1 million fewer people live in South Dakota).
Tip: This to-scale comparison of Senegal vs. South Dakota uses the Mercator projection, which distorts the size of regions near the poles.