Western Sahara is about 1.5 times bigger than Missouri.

Missouri is approximately 178,414 sq km, while Western Sahara is approximately 266,000 sq km, making Western Sahara 49% larger than Missouri. Meanwhile, the population of Missouri is ~6.0 million people (5.3 million fewer people live in Western Sahara).
Tip: This to-scale comparison of Missouri vs. Western Sahara uses the Mercator projection, which distorts the size of regions near the poles.