What is the world's largest volcanic island?

Iceland.

https://en.wikipedia.org/wiki/Island#Oceanic_islands

Sumatra, Indonesia - (171,069 sq miles) (443,066 sq km)
Honshu, Japan - (87,182 sq miles) (225,800 sq km)
Java, Indonesia - (53,589 sq miles) (138,794 sq km)
North Island, NZ - (43,082 sq miles) (111,583 sq km)
Luzon, Philippines - (42,458 sq miles) (109,965 sq km)
Iceland - (39,769 sq miles) (103,000 sq km)
Mindanao, Philippines - (37,657 sq miles) (97,530 sq km)
Hokkaido, Japan - (30,395 sq miles) (78,719 sq km)
New Britain, PNG - (13,569 sq miles) (35,145 sq km)


http://worldatlas.com/aatlas/infopage/islands.htm
Halmaherea, Indonesia - (6,965 sq miles) (18,040 sq km)

Iceland may be the Largest ACTIVE Volcanic Island.

I believe Java is the right answer. Sumatra and several others at worldatlas.com aren't strictly volcanic. They have volcanoes but weren't formed by volcanic eruptions.

http://www.worldislandinfo.com/SUPERLATIVESV2.html

The above source lists Iceland, but Java is larger and was formed mainly by volcanic action.

https://en.wikipedia.org/wiki/Java

Is there a geologist in the house?

Maybe Iceland is not such a bad answer anyway. Quote "Java is almost entirely of volcanic origin". Which island, then, is the largest that is ENTIRELY of volcanic origin? North Island, NZ, is also larger than Iceland. Does anyone know about the origin of this island?

Hmmm, I thought I'd gotten back to this....
I spoke with a geologist who knew generalities but not specific square mileages and formations.
Looks like Malarson's original answer was correct, and his link deals with the different kinds of islands.

It's hard to find specific data on North Island, but New Zealand itself was not mainly formed by volcanic action.

http://data.gns.cri.nz/geoatlas/text.jsp