The room has to be at least 4m by 4m to make an island workable, and in an L or U shape kitchen you will need a 1m to 1.2m gap between island and worktop. In an open plan configuration, the island will delineate the kitchen from the other areas. So you may want to make your island stand out with a different or bolder colour/finish.
The first thing to do when you design your island is decide what you will use it for: prep, storage, seating. You can even include a drop down table. I wouldn’t choose to have my hob on the island, to avoid splattering guests who might be sitting at it while I cook. But with downdraft extractors, and windows taking up a lot of wall space, it’s become a viable option and it’s now quite commonplace.