Missoula is about 10 times smaller than Dallas.

Dallas is approximately 880 sq km, while Missoula (map shows its 17 named neighbourhoods; the area covers the whole city) is approximately 91 sq km, making Missoula 10.31% the size of Dallas.
Tip: This to-scale comparison of Dallas vs. Missoula uses the Mercator projection, which distorts the size of regions near the poles.
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