Hello. I have a 25 mo who is currently a great sleeper in the cot. He has not started to stand up or attempt to climb out of the cot. However, he is 88cm tall and I heard once a toddler reaches 89, they are considered too tall and unsafe to sleep in cot. Is this true?

I am not aware of this recommendation?

Red Nose recommendation for moving from a cot to bed is
Red Nose recommends that you move your child from a cot to a bed when they have attempted to climb out of their cot & look like they will be successful.
This usually occurs between 2yr - 3.5yrs but may be as young as 18mths.
https://rednose.org.au/article/where-to-sleep-a-child-who-has-moved-from-a-cot
https://rednose.org.au/article/making-your-home-safe-before-moving-a-child-from-a-cot-to-a-bed