In this new update, large islands can be found simply through drifting and coming across one like you would a normal sized island. However, the original intention with this achievement was to have the player use their receiver, after find the Radio Tower (Utopia), in order to locate a large island. It can take a lot of patience to find a large island by chance so if you have the receiver built you can plug a battery into it and it will show you where to go.
The large islands will appear as green dots on the map while the blue marker will be a story location based on whatever coordinates you have entered into the right module of the receiver. Just go to one of these green dots and you will get this achievement.