Flathead Lake is about 1.2 times smaller than Tokyo.

Tokyo is approximately 619 sq km, while Flathead Lake is approximately 510 km² (197 sq mi), making Flathead Lake 82.39% the size of Tokyo.
Tip: This to-scale comparison of Tokyo vs. Flathead Lake uses the Mercator projection, which distorts the size of regions near the poles.
Our free app, MapSize by MyLifeElsewhere — compare any two places, to scale on your mobile device.