The Island as it appeared in Season 1(top) and Season 2.(bottom)
Gilligan's Island is a typical South Pacific tropical Island, with beaches, mountains, jungles, and a lagoon. It is also the location of an active volcano and was the target of a USAF experimental missile. Luckily, the explosives failed and it did not explode. The islands position is said to be approximately 110 longitude by 10 latitude. The neighboring islands are inhabited by headhunters and voodoo witch doctors. Some of the natives are friendly, including a rather obese woman who wanted to marry Gilligan and a girl named Kilani who Gilligan saved from drowning who wanted to become his slave.
Map of the island from Episode - Forward March.
The Professor makes a map to track a typhoon due to hit the island. (Episode - Hi-Fi Gilligan)
huts exhausted after exploring the island. However, in the season two episode "Forward March", the professor and the skipper draw up a map of the island and it appears quite small. And still the shape of the island in the latter map differs from it's shape in a map in the episode "Hi-Fi Gilligan". Also, differant real-life islands are portrayed as being the island in the first season's theme song and that of the second season, as well as in different episodes. So, based on these facts the size and shape of the Island is not clear.
Another view of the island from the second episode, Home Sweet Hut.