Amsterdam is about 4.7 times smaller than New York City.

New York City (including all five boroughs: Manhattan, Brooklyn, Queens, The Bronx, and Staten Island) is approximately 781 sq km, while Amsterdam is approximately 166 sq km, making Amsterdam 21.22% the size of New York City.
Tip: This to-scale comparison of New York City vs. Amsterdam uses the Mercator projection, which distorts the size of regions near the poles.