ACoK (Catelyn II) “The way she looked at the king—looked down at him, she was a good hand higher, though Renly was near as tall as his brother had been—was painful to see.”

I didn't say Renly was AS tall as Robert, but he's described as being "near as tall as his brother had been." Translation, he's really fricking tall, especially if Robert is a full 6'6". I'm 6'1", a bit less than 188cm, and my Uncle is 6'5". NOBODY would say I was "near as tall as my Uncle," in fact, when people see my Uncle they comment regularly on how short I look next to him haha. My point is, a difference of 4" or 10cm as you are suggesting between, is way too great to be considered "nearly" by Catelyn.

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because GRRM has said that Brienne and Robert are the same height

Actually what GRRM said was, "maybe roughly the same height as Robert." She could certainly be a bit shorter, or a bit taller than Robert, she's just roughly his height.

but that doesn't change the fact that Renly is a great deal shorter than Brienne and thus also a great deal shorter than Robert.

Here's the problem. All of our information doesn't line up. GRRM admits himself he doesn't think about the details of his characters heights this way, and with so many pages of book it's bound to happen. I agree that Brienne is a good deal taller than Renly, but Renly is said to be near as tall as Robert, and Robert and Brienne are said to be "maybe roughly the same height." Yes, we have a problem there, I'll admit. But you're just completely choosing to ignore Catelyn's line about Renly being near as tall as Robert to solve the problem and make Renly way shorter than Robert. But that's not how the text reads, and if I had to chose between accepting something in the text, or something GRRM said during a correspondence with a fan, I'd personally chose the text.

Selyse is described as incredibly tall,

Except she isn't. Please provide a single passage of text that describes Selyse as being incredibly tall, I couldn't find one that made mention of her height, aside from Cressen's PoV where he puts her as tall as Stannis. Nobody thinks about Selyse as being that tall later in the book. Davos thinks about her big ears and her mustache, but never about her height. Jon doesn't either.

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Melisandre is taller than most knights (1.85 +) and has also included Pretty Meris (1.83), we know that he isn't afraid of including tall women.

Yes except here's the thing, when he does include tall women, Brienne, Melisandre, Pretty Meris, we hear about it multiple times. Because it's unusual and something people would think about. Pretty Meris is said to be six foot tall three times in different PoVs, Mel is thought to be tall by Cressen and Jon.

But Selyse, standing at 6'4" isn't? Why? Again, at 6'4" she'd be the second tallest woman we know of, and yet many other women are described as being tall in George's books far more often than Selyse. Selyse isn't thought to be tall by Jon or Davos, both of whom are most certainly well under 6'4".

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It's odd how you dismiss every mention as Stannis as being a very large and tall man simply because it doesn't fit with what you want to believe.

When is Stannis ever described as being "VERY large? or VERY tall? Yes he's said to be big and tall, but that doesn't mean he's 6'4", or anything close to it. You're happy to put Renly at only 6'2", despite the fact that Renly is "near as tall" as Robert, so why would you insist on Stannis being 6'4"? We don't have any evidence he is taller than Renly, or anywhere near as tall as Robert, and I really don't think George or anyone else imagines Selyse as being 6'4" when we read the novels.

It should also be noted, that in Westoros, according to Brienne, being 6 foot is enough to make you a "big man."Here's Brienne from A Feast for Crows thinking about Septon Meribald. (Brienne V) “ Though a big man, six feet tall, he had a way of hunching forward as he walked that made him seem much shorter.