Guernsey is about 2.3 times smaller than District of Columbia.

District of Columbia is approximately 177 sq km, while Guernsey is approximately 78 sq km, making Guernsey 44.07% the size of District of Columbia. Meanwhile, the population of District of Columbia is ~601,723 people (534,232 fewer people live in Guernsey).
Tip: This to-scale comparison of District of Columbia vs. Guernsey uses the Mercator projection, which distorts the size of regions near the poles.