I read this one as a child and I've never been able to find it again! It wasn't exactly Pulitzer material lol, but I have nostalgic memories of it.
I believe the author was an American woman. It was published in the 90s at the latest, possibly 70s-80s. The title possibly had something like 'sorceress' in it. I believe the book cover had a picture of a sorceress in ragged black, wind-whipped clothing (I have a vague impression of a greyish-blue background).
A nobleman (a duke?), described as handsome and dark-haired, gets drunk and rowdy in an inn. He humiliates a fellow guest who has deformities. The guest turns out to be a(n impoverished?) king, who's also a magician, so he places the nobleman under a curse.
The curse is that the nobleman becomes uncontrollably hungry, and as he eats he becomes a giant. After eating so much that he's cleared out the townsfolks' larders, can't foot the bill, and reaches an enormous height, he's chased away.
He meets a chipper lad, who becomes his sidekick, and hides in a field of magical (and unfriendly!) plants. Then he and the sidekick set off to find the witch-king and beg him to lift the curse.
After some danger and adventures, they find the gloomy draughty castle. The witch-king agrees to slow down, but not lift, the curse-- if the nobleman works in his kitchen as a scullery boy. The witch-king says he'll review the punishment... in a few decades' time.
Eventually, the noble-turned-scullery-boy meets the witch-king's predictably beautiful daughter. (Blonde, obviously. Described as pretty as a spring morning!) She's sweet but sad, she's nice to him, and they become friends.
Then tragedy strikes! The witch-king's disabling and painful deformities, it's revealed, were the result of a sorceress' curse on his line many centuries ago. His daughter is also becoming sick. Her limbs will become painful and twisted until she, too, must endure the rest of her life in agony. Or maybe die?
The king announces he's sending off an adventuring party on a quest to find the sorceress. She was capable (he thinks) of extending her life indefinitely. And they'll find her in a ruin in the middle of a swamp. Whoever finds her will be promised his daughter's hand in marriage (yuck, but again, not Pulitzer material).
So some bold-yet-unpleasant noblemen, the giant scullery-boy, and the chipper sidekick set off in a magical sledge into the swamp. It's dangerous, horrible, they're plagued by insects, and most of them die.
Eventually they find the sorceress in her ruin. She's staring at some magical paintings which keep her in a state of torpor. They break her torpor. For some reason, she agrees to come back with them and cure the king's daughter.
The cure involves the sorceress magically replacing all of the princess' blood with the giant nobleman's (hope he's Group O). This means he will die, to which he agrees in order to save her.
However, in a bold and unpredictable twist, he wakes up again a bit later. The witch-king tells him he had just enough blood left to survive, if he was the size of a normal man. So the witch-king broke the curse. Marriage, redemption etc, happily ever after.
I realise I remember the entire story so well, finding the book seems redundant... but it's been a mystery for me for 20-ish years! I began to suspect I'd imagined this book!