So, I've been messing around with HeightComparison.Com (LINK) recently in order to get a feel for my own headcanons about the Arthurian character's heights, as well as help visualize the scale of the Arthurian giants.
The following is an experiment and a mix of both hard info from the texts and my own imagination. For the latter, I've tentatively set up a couple of assumptions:
King Claudas will be my yardstick, being described as 9 ft. tall and not directly referred to as giant, merely as a very tall man. A male (in red) of average height at 5 '9 ft. will also be shown for scale.
Mixed-blooded giants and half-giants do not go above 12 ft.
First, King Arthur and Queen Guinevere will be placed at 8' 8 ft. and 12 ft. respectively. I personally want Arthur to be tall - taller than Lancelot - in respect to the folk myths that portray him as a giant but also not wanting him to exceed Claudas. I also headcanon Uther Pendragon as being around the 8 ft. range as well. Guinevere's height is set at the max for what I feel is the tallest a half-giant(ess) could be, just for an amusing height difference between her and Arthur. Assume Guinevak is also at this height.
[...]
Now here are two giants King Arthur fights in the Vulgate, with Gwen and an average man for scale. Already, we can see how crazy the dynamics are for Arthur's battles (as well as the other giant slayers). Arthur is already impossibly tall for a man but he is still basically a little kid next to Rion and Caelenc, albeit a little kid armed with a functional lightsaber but still...
Next, we get into the love triangles. Lancelot-Guinevere-Galehaut (plus Arthur) and Tristan-Iseult-Palamedes. Galehaut, Palamedes and Iseult are known half-giants, so I've assigned them some numbers close to Guinevere's. Galehaut is described as "six inches taller than any knight", so I gave him the advantage over Palamedes. Lancelot and Tristan, as I visualize it, I've decided to be around a foot taller than Gawain, who I made pretty short by comparison (see below). So they're seven footers here. This is comparable to Yao Ming of NBA fame (7' 6 ft.) This still leaves both Tristan and Lancelot as the shortest members of each of their respective love triangles. XD
Here is an ensemble of male characters to compare Gawain with. One of the interesting descriptions about Gawain (in Vulgate) is that he is "neither too tall or too short" and is physically balanced compared to his brothers. I haven't hammered down the other Orkney brothers yet, so they're not here. As a result, I hilariously made Gawain the shortest of this roster at a measly... 6 '9 ft. So yeah, even though he's the shortest, Gawain is still decently tall compared to an average male, just the most realistic of the bunch.
And here are the ladies. The ex-gfs and their rebounds, plus Nimue Also, sort of a giants vs. fairies thing, with that poor human caught in the middle. Morgan, I feel, just deserves to be the tallest of all the female fairies, just four inches below her brother.
Next, I have a sequence of giants prepped, starting with Galehaut the half-giant and King Rion:
Now we add Gogmagog, the giant from Historia Regum Britanniae, who was defeated by Brutus and Corineus. He is described as 12 cubits tall (18 feet):
Then we factor in Sir Kay and his size-changing powers at max. Supposedly, he can grow "as tall as the tallest tree in the forest". Let's make an arbitrary choice and assume Kay can grow as large as an oak tree (specifically, a Sessile Oak) Which means THIS is Kay's theoretical max height:
And finally, let's bring in Bran the Blessed, the legendary giant king of Welsh Myth, who waded across the Irish Sea and was seen as massive object by the Irish. Which I have calculated here. (Note: the given number of 262 ft. is a lower estimation of Bran's total height)
To wrap up this already long post, here's two more rosters i wanted to show: the first being the regulars of the Arthurian cast in contrast. The second is Arthur, Guinevere and their children, based on how it feels in my conception.
*(Duran is the shortest because he's the youngest in my HC. He would probably be taller had he not died at Camlann. Similarly, my HC Loholt was killed before he was 25. Amhar, on the other hand, can change size)
EDIT: For additional info on Arthurian giants, see my post on known giants and their stated heights.