- Missoula County, Montana has 2,593.0 square miles of land area and is the 25th largest county in Montana by total area. Missoula County, Montana is bordered by Sanders County, Montana, Powell County, Montana, Mineral County, Montana, Ravalli County, Montana, Lake County, Montana, Flathead County, Montana, Clearwater County, Idaho, Idaho County, Idaho, and Granite County, Montana.