Las Vegas is about 2.1 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 Las Vegas is approximately 367 sq km, making Las Vegas 47.04% the size of New York City.
Tip: This to-scale comparison of New York City vs. Las Vegas uses the Mercator projection, which distorts the size of regions near the poles.