South Dakota is about 1.4 times smaller than New Zealand.

New Zealand is approximately 268,838 sq km, while South Dakota is approximately 196,540 sq km, making South Dakota 73.11% the size of New Zealand. Meanwhile, the population of New Zealand is ~5.1 million people (4.2 million fewer people live in South Dakota).
Tip: This to-scale comparison of New Zealand vs. South Dakota uses the Mercator projection, which distorts the size of regions near the poles.