How big is a full grown shichi dog?

There is no such thing as a "Shichi dog". It is possible that you have misspelled the name of a breed, or that you are referring to a fictional dog breed.

Please provide more information about the dog you are asking about, such as:

* The full name of the breed

* A description of the dog's appearance

* Where you encountered the name "Shichi dog"

With more information, I can help you find the answer to your question.