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 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)
a few times the professor is seen arriving back at the 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 in the episode "X Marks the Spot" the islands position is said to be about 140 longitude by 10 latitude. However in the episode "Big Man on a Little Stick" the Skipper tells surfer Duke Williams to tell people on the mainland that there position is approximately 110 longitude by 10 latitude. Different real-life islands are also 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.