It is a very ambiguous question, and, to toss something else into the mix, the island comprising England, Scotland and Wales ( i.e. Great Britain ) is the largest of the British isles.
As Chelle7272 states though, if it a reference to what we call the British Isles ( i.e. Great Britain and Ireland ) the answer is Isle of man.
It is very much a question of interpretation. The British Isles are generally defined as Great Britain, Ireland and various other smaller islands such as the Scillies, Isle of Man, Orkneys etc.
My own interpretation of the question - and I'm sure that others will disagree - is that only the two largest islands have been "included" in the question and that what is being looked for is the largest of the smaller islands, that being the Isle of Man..
Apologies that I can't be of any further help than that.