Serbia is about 1.4 times smaller than Cuba.

Cuba is approximately 110,860 sq km, while Serbia is approximately 77,474 sq km, making Serbia 69.88% the size of Cuba. Meanwhile, the population of Cuba is ~11.0 million people (4.3 million fewer people live in Serbia).
Tip: This to-scale comparison of Cuba vs. Serbia uses the Mercator projection, which distorts the size of regions near the poles.