It was thought that the "island" has the size of 5,000,000 km². After reading a bit about geography, I'm unsure. My island would be far bigger than even the largest islands on Earth. But it would still be much smaller than the continents on earth. Now I'm undecided. Would the size still be fine for an island or should I call it a continent?
Your island simply needs to be part of a big Continental plate You will notice that Greenland is part of the North american plate and therefore an island. But India can be called a sub-Continent because it is on its own plate. The movement of these plates is what typically causes earthquakes. So the African plate is moving with about 1 cm per year towards Europa. Whilst the British isle's are still in the same spot in relation to mainland Eurasia when a ice age separated them. Even though erosion of the coast might suggest otherwise.
So if you need a large island have there be a larger piece of land than you island nearby. And the sea or strait between them should have no underwater Vulcan's or earthquakes near the coastal regions.
So it all depends on whether you care about having small volcanic island's between your island/continent and its mainland or not. Most people wont care much as long as there does exist a bigger piece of land mass close by. or lots of same sized big islands.
There is a continuum between an islet (a tiny island) and Eurasia (the largest continent.)
There is no scientific distinction that makes Europe and Asia separate continents and makes Australia a continent and Greenland not. (Geology speaks of "continents", but the definition is different from the geographical one.)
The geographical term is an arbitrary distinction which is never-the-less useful in communication. So if you want to stress how large it is call it a continent, and if you want to stress how isolated and limited it is, call it an island. (Whatever you do, don't call it a continent and then call larger land masses islands or call it an island and then call smaller land masses continents.)
It's all about communication, so choose your word to communicate what you want to communicate.
It's still a continent. A good borderline is 1/4 the size of Australia