Yep, they're all human (more or less):
Giant: Barak of Cherek. His people are that world's generic ancient Danes (think Beowulf)/Scandinavians. They are supposed to be huge; some of them were around 7 feet tall. Barak is supposed to be "giant", so I imagine he is over 6'5"
Rat-man with the coins: Prince Kheldar of Drasnia. He's human, but the way he is described in the books does not really match what he looks like on the cover. He is supposed to be "small and wiry", but Eddings mentioned somewhere in his published notes that Kheldar is supposed to be around 5'5" or so.
Coat Rack: C'Nedra, Imperial princess. Human (sort of). Technically she's a Dryad, although her father was human; apparently all dryads are female, and all female children of a dryad and a human are pure-blood dryads. No idea what male offspring are - probably human. She's supposed to be like 5'0" and 95lbs. Also a red head with curly hair.
Gandalf-looking mofo: Belgarath the Sorcerer. A seven-thousand year old human and arguably one of the most powerful beings in literary fantasy. Apparently he is able to control the weather across an entire planet, fuck with time, tear mountains open and level entire cities. But doesn't, because while an asshole, is still generally a good person. He is not supposed to look like that guy.
Kid with the pointy stick: Belgarion, Godslayer, Overlord of the West, King of Riva, Lord of the Western Sea, Child of Light, etc. etc. etc. While holding that sword, he possesses limitless (and I do mean limitless) power. There is fundamentally nothing he cannot do. He can kill gods, resurrect people, annihilate all life in the universe (apparently). In that picture he's supposed to be about 16 years old and about 6'2".
I've read this series so many times I've worn out multiple copies of each book. When I was younger I didn't have the happiest of lives, and this series was the way I escaped it. Looking back on the series now, they are not innovative or even original, but as Eddings wrote in his "Rivan Codex", they were never meant to be innovative or original. They were formulaic, because the formula sold books.