Answer:
It is a detail.
Step-by-step explanation:
The sentence "Our dog is only a few months old and can already reach the kitchen counter when she stands on her hind legs" is a detail that supports the topic sentence "Our puppy will probably grow into a large dog."
The fact that the puppy is already tall enough to reach the kitchen counter at a few months old suggests that it has the potential to become a larger dog in the future.
This detail helps to strengthen the argument that the puppy will probably grow into a large dog.
Hope this helps!