Montana is about 1.4 times bigger than New Zealand.

New Zealand is approximately 268,838 sq km, while Montana is approximately 376,979 sq km, making Montana 40% larger than New Zealand. Meanwhile, the population of New Zealand is ~5.1 million people (4.1 million fewer people live in Montana).
Tip: This to-scale comparison of New Zealand vs. Montana uses the Mercator projection, which distorts the size of regions near the poles.