Do all Nightmares (including husks) grow bigger the older they get?

The answer is: yes

However this depends greatly on the Nightmares hunting Style, as a Nightmare gets older and more powerful their corruption does twist their frame more and more, usually resulting in a bigger frame

However a ton of different factors result in a Nightmares hunting Style, their size, their title, and eventually their tormented form.

Malice is the oldest Nightmare and the biggest but he not the only ancient, there are total of 6 intelligent ones and over a couple hundred husks, which all of their sizes vary on age and hunting Style.

Husks are generally large no matter what, some even get close to Malice but that because their energy goes into far less things than intelligent Nightmare dose. Husks only eat and sleep, nothing more nothing less ( besides going to the bathroom) so most of their energy just goes into becoming bigger to eat bigger things.

As an intelligent Nightmare as far more things to place the energy they have to buff up things.

For example: Tainted dream is older then Lamia but she smaller in size, this is because the two hunting style are vastly different, Lamia relies on her brute strength and dirty tricks meaning her corruption would change her frame to better suit that hunting style, giving her a bigger frame as Tainted is still big been a ancient but she isn't really big either because her hunting style is more Cloak and Dagger, a bigger bulkier frame would be more of a inconvenience to her hunting Style then a smaller faster one, because of her smaller size Tainted can go far more places in her true form then Malice and Lamia could.

Another example: Malice might be the biggest Nightmare but he isn't the bulkiest, that would be Tom or some of the Husks, if you stripped away the title of King Mal is more of a balanced out Nightmare, his former self was a warrior, one who was more balanced out as Tom's former self was already bulky and more of a muscle warrior, as a Nightmare the corruption only amplified this as Tom's hunting style is very reckless and barbaric, his corruption would adapt his frame to suit the constant abuse it takes, making him one of the bulkiest ancients even though he is the second youngest.

Sometimes bigger isn't always better but it all depends on the Nightmare hunting Style.

What's the max height for a Nightmare in his/her true form?

Answer: unknown

It's unknown what the true height of a Nightmare can reach as Malice is the oldest Nightmare and he still growing.

Even though in the land of dreams Nightmare to the present-day people seem like they've been for around forever, but that not the case, Nightmares I actually a young species at roughly 20,000 years old, their still a lot of unknowns about them, how big they get and how long can they really live are all unknown.

That all on this one