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.
Presumably the Island is quite large, based of the fact that there are features on other parts of the island that are foreign to the parts near hut area. Also it is mentioned that the island has jungles, not a jungle.
Another view of the island from the second episode, Home Sweet Hut.
on. Such features include wild animals, quick-sand, and (as mentioned above) an active volcano. Also, a few times the professor is seen arriving back at the huts, after exploring the island, out of breath. And yet, in the season two episode "Forward March", the professor and the skipper examine a map that they drew up of the island and it appears quite small. So, based on these facts the size of the Island in unkown.