Jacksonville is about 2.8 times smaller than Istanbul.

Istanbul is approximately 5,343 sq km, while Jacksonville (map shows its 197 named neighbourhoods; the area covers the whole city) is approximately 1,935 sq km, making Jacksonville 36.22% the size of Istanbul.
Tip: This to-scale comparison of Istanbul vs. Jacksonville 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.