It is not easy to determine what the second biggest island is. There is no agreement as to when a landmass becomes a continent or is just an island. For example, Australia is often called a continent. However, it can also be labeled as an island.
Islands Larger than 250,000 Sq Km
Using this criteria, the second largest island would be New Guinea. It has an area of 785,753 sq km or 303,381 sq mi. The biggest would be Greenland which has an area of 2,130,800 sq km / 822,706 sq mi. Number three would be Borneo. It has an area of 748,168 sq km or 288,869 sq mi.
At number four would be Madagascar. It has an area of 587,713 sq km or 226,917 sq mi. Fifth would be Baffin Island. Its total area is 507,451 sq km or 195,928 sq mi.
If Australia is included in this list, the second biggest island would be Greenland.
Islands between 25,000–250,000 Sq Km
If this criteria is applied, the second largest island would be Victoria Island (Canada, Northwest Territories and Nunavut). It has a total area of 217,291 sq km or 83,897 sq mi. The largest would be Honshu, Japan at 225,800 sq km or 87,182 sq mi. Great Britain would be ranked third with an area of 209,331 sq km / 80,823 sq mi.
Ellesmere Island (Canada) would be number four. It has a total area of 196,236 sq km or 75,767 sq mi. Sulawesi (Indonesia) would be the fifth largest. It has a total area of 180,681 sq km or 69,761 sq mi.
Islands between 10,000–25,000 Sq Km
The second largest would be Kotelny/Faddeyevsky Island, Russia. It measures 24,000 sq km or about 9,266 sq mi. The biggest would be Somerset Island, Canada. It has a total area of 24,786 sq km or 9,570sq mi.
Number three would be Sardinia, Italy. It is 23,949 sq km / 9,247 sq mi. At number four would be Bananal, Brazil. Its total area is 20,000 sq km or 7,722 sq mi. In fifth place is Shikoku, Japan. It has a total area of 18,545 sq km or 7,160 sq mi.
If the definition of island is a landmass between 5,000–10,000 sq km, the second biggest island would be Prince Charles Island. It measures 9,521 sq km / 3,676 sq mi. For islands between 500–1,000 sq km, the second biggest is Santiago, Cape Verde at 991 sq km / 383 sq mi.