Montana is about 2.1 times bigger than North Dakota.

North Dakota is approximately 178,647 sq km, while Montana is approximately 376,979 sq km, making Montana 111% larger than North Dakota. Meanwhile, the population of North Dakota is ~672,591 people (316,824 more people live in Montana).
Tip: This to-scale comparison of North Dakota vs. Montana uses the Mercator projection, which distorts the size of regions near the poles.