North Dakota is about 2.1 times smaller than Montana.

Montana is approximately 376,979 sq km, while North Dakota is approximately 178,647 sq km, making North Dakota 47.39% the size of Montana. Meanwhile, the population of Montana is ~989,415 people (316,824 fewer people live in North Dakota).
Tip: This to-scale comparison of Montana vs. North Dakota uses the Mercator projection, which distorts the size of regions near the poles.