Irving doesn't provide much physical description of Rip, focusing instead on his character, which is happy-go-lucky, beloved of children, and lazy. We know the young Rip is strong enough to carry children on his back and that the women of the village take his side against his scolding, nagging wife, which might suggest he is decent looking--in any case, it suggests he is a pleasant enough fellow and that his wife is not well liked. Rip tends to stroll about rather than move at a fast pace, he carries a gun with him, and he wears old clothes because it doesn't much matter to him what he puts on as long as he doesn't have to work too hard.
After he sleeps for twenty years, he is stiffer than he was before, and his beard has grown a foot long and turned gray.
Irving gives us much more detailed descriptions of peripheral characters, such as the magical people in the woods who send Rip to sleep for 20 years, than he does of Rip. Irving seems to have done this on purpose, deliberately leaving it up to readers to conjure their own visions of how the young and old Van Winkle would have appeared. This is not unusual for the period: Jane Austen also often leaves it up to the reader to supply the specific details of the appearances of her main characters, such as Darcy and Elizabeth in Pride and Prejudice.