Japan is roughly the size of Montana.

Area in square miles:

California: ~ 164k

Montana: ~ 147k

Japan: ~ 145k

N. Mexico: ~ 122k

...

Nebraska: ~ 77k - FYI Nebraska is a terrible answer. It is about half the size of Japan

So, the answer is Montana, but people are more familiar with California. Also, Japan and California are vertically oriented, which might make for an easier comparison.