Twain doesn't provide a description of the types of characteristics you are asking about. Jim is first mentioned in the story as being "Miss Watson's big nigger" but other physical features have to be educated guesses. Because he worked in and around the house, he was presumably strong and well-built. If he had been sold down the river, Jim was aware that "I's wuth eight hund'd dollars", which supports the assumption that he was in prime physical condition. He was married and had children, so was possibly in his twenties or thirties.
Jim's appearance changes through the story as he and Huck don disguises to get out of various scrapes, as they enjoy swims in the river, and as they expand their wardrobe with clothing from the house that is washed down the Mississippi.