Answer to this question is not easy. Do you mean island in general or it has to be country-island?
Here is a list of islands by area .
It is easy to say what is the biggest island (except continents). That would be Greenland, probably. But we don't know how many little, tiny islands are there in the world, somewhere in the middle of the oceans and seas.
I found this information.
In Croatia where I live are dozens of small islands. For example:
Island of life - Wikipedia article is only written in Croatian. It says that is big only 0,011
Galijula - also written only in Croatian, smaller that Island of Life, only 0.003
If you need more information about this islands I will try to translate it to English.