The section of this great novel that you want to look at is Chapter Twelve, which describes how Jim and Huck fled the island and the people eager to earn some money by capturing Jim, the runaway slave. As they make their way downstream, we are told that Jim used some fo the top planks of the raft to build a wigwam for protection from the elements. He made a floor for the wigwam and raised it so that everything would remain dry from waves coming from other boats. Because of Jim's ingenuity, they could have a fire on the raft that could be hidden. They made an extra oar and also "fixed up" a short stick on which they could hang a lantern to advise other ships of their presence.
In Chapter Nine we are given the measurements of the raft. Note what Huck tells us about it:
One night we catched a little section of a lumber raft--nice pine planks. It was twelve foot wide and about fifteen or sixteen foot long, and the top stood above water six or seven inches--a solid, level floor.
Hopefully these two descriptions of the raft will help you imagine how Jim and Huck lived on it for so long, and how it became their home.