Because Connell does not explicitly describe Sanger Rainsford, readers must construct their own ideas about how he looks. He is obviously a wealthy man who is able to travel the world on hunting expeditions. He is a New Yorker. He carries himself confidently, and aboard the yacht, he tells Whitney, "The world is made up of two classes—the hunters and the huntees." He knows how to relax and enjoy a good pipe, which is what he is doing just before he falls overboard.
When Rainsford hears the shots from Ship Trap Island, "he leaped upon the rail and balanced himself there" on the yacht; this tells us that he is athletic and agile. When he swims to the island, he does so with "strong strokes," and when he awakens on the beach, "he looked about him, almost cheerfully." He is intelligent, blessed with keen eyesight, and is mentally and physically strong.
When he meets Ivan, he offers "a smile which he hoped was disarming." This tells us that Rainsford possesses social intelligence and knows how to manipulate people.
There is another clue about Rainsford's physical look; Zaroff is described as a "slender man," and when he gives Rainsford his bespoke suit, it apparently fits him. Readers can extrapolate that Zaroff and Rainsford are not only a match intellectually (though certainly not morally), they are a physical match, as well. However, in terms of facial features, namely, eye, skin, and hair color, Connell offers no clues.