depends on wind conditions onshore, and distance to grid. There is no size that seems more advantageous than any other.

Offshore, you have large fixed costs in the form of the cable to the grid, and the transformer station. So, in principle, the larger the better - at least 200MW, and more if, like in Germany you need to go quite far from the shore.

In the long run, we're all dead. John Maynard Keynes