Tonyblack wrote:Isn't there a description of her appearing taller than she actually is - somewhere?
That's later, in the Tiffany books.
People do often get slightly shorter as they age and the padding between the vertebrae gets thinner. However, I'm fairly certain that pTerry originally intended Granny to be quite tall and then decided to change it to "appears" tall; he did that first with the head schoolteacher at Susan's school in SM, and (if I recall correctly, and I think I do) he has Susan thinking of her as "the woman" which is a way of reducing someone's influence.
Other people in Ankh-Morpork definitely perceive Granny as tall, and she was perceived as fitting into a tall woman's dress in WA. So Granny is actually at least medium height for the Ankh-Morpork average; Nanny may well be short.
However, Tiffany first met Granny when she was only nine years old, and she may have perceived Granny as taller than she really was. Later, when Tiffany is older, taller, and less nervous about Granny, she changes her emotional perception and now sees her as less tall than she originally thought. This new perception may not be any more accurate than the first one. Tiffany now has emotional reasons for wanting to see Granny as less imposing. All the first sight in the world isn't going to combat a deep emotional need.