San Francisco is about 3.1 times smaller than Birmingham.

Birmingham (map shows its 87 named neighbourhoods; the area covers the whole city) is approximately 381 sq km, while San Francisco (including the city proper only) is approximately 121 sq km, making San Francisco 31.77% the size of Birmingham.
Tip: This to-scale comparison of Birmingham vs. San Francisco 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.