Samui A district that has a group of islands in the Gulf of Thailand in Surat Thani province, one in three is a flat area surrounded by mountains. The major attractions of the Gulf of Thailand. Samui Island area of approximately 252 square kilometers, as well. Koh Samui has its own unique area of about 228 square kilometers. It is the third largest island of Thailand after Phuket and Koh Chang.