Goliaths debuted in the 3.5e supplement Races of Stone (2004), which does indeed go into more detail about them, since it has a 32-page chapter devoted to them.
Physical description: A typical goliath is larger than the largest half-orc. Most stand between 7 and 8 feet tall and weigh between 280 and 340 pounds. Unlike with most other races, there is no appreciable difference in height or weight between male and female goliaths.
Goliaths have gray skin mottled with dark and light patches that goliath shamans say hint at a particular goliath’s fate. Lithoderms—coin-sized bone-and-skin growths as hard as pebbles—speckle their arms, shoulders, and torso. Their skulls have a jutting eyebrow ridge, wide jaw, and occasional lithoderms as well. Female goliaths have dark hair on their heads, grown to great length and always kept braided. Male goliaths generally have hair only on their limbs. Goliaths’ eyes are a brilliant blue or green, and they often seem to glow a little from underneath their furrowed brows.
Because their skin mottling has cultural significance, goliaths generally dress as lightly as possible, displaying their skin patterns for all to see. For the same reason, few goliaths would willingly get a tattoo—to draw on one’s skin is tantamount to trying to rewrite one’s fate. Goliaths instead decorate themselves with jewelry, often sporting ear, nose, or brow rings. A goliath’s lithoderms are also common places to embed a gem or two, since they have few nerve endings and stand out on the goliath’s body already.
(Races of Stone pg. 54)
The Races of Stone art gallery has many images of goliaths:
Many of the chapter headings also include goliaths, but since they are line drawings intended to cover the top half of a page, but quite small in the gallery, and don’t focus particularly on goliaths, I haven’t linked them specifically. Races of Stone’s cover also depicts a goliath.
The lithoderms were a big part of goliaths’ original description and illustrations; every edition since has left them out of both descriptions and illustrations. Make of that what you will—I suspect this was done purely to make goliaths look more appealing to (presumed-human) players, since the lithoderms might look ugly or even diseased to a human. Personally, that seems kind of disappointing, as they were kind of unique, had neat tie-ins with their culture, and made goliaths something more than just “big people”—and despite the fact that I, too, find them a bit off-putting.
Also, note that the goliath weight range, which has gone unchanged from 3.5e to 5e, seems dubious given goliath illustrations. For what it’s worth, though, 3.5e illustrations do seem to fairly often depict NBA-like physiques, rather than Hafþór Björnsson-esque physiques (though most of them are definitely more heavily muscled than Yao Ming), making the weights maybe somewhat closer to accurate. Even in 3.5e they should probably be quite a bit higher than they are.