London is about 3.4 times bigger than Northern Mariana Islands.

Northern Mariana Islands is approximately 464 sq km, while London (including the 32 boroughs of greater London) is approximately 1,572 sq km, making London 239% larger than Northern Mariana Islands.
Tip: This to-scale comparison of Northern Mariana Islands vs. London uses the Mercator projection, which distorts the size of regions near the poles.