Stack height design (Boiler)

Dear all,
how can we design and decide the stack height for boiler. also be consider the diameter of it.

1200 mm is the required dia of 30 meter Chiemeny. This is for larger boiler like 1 ton and above this.

Dear @Ashwani Bhai our existing boiler chimney height is 42 feet and diameter is 18 inch, so any guideline for boiler chimney height reference. also be selection of boiler chimney height as per selection of boiler ton.
Advance appreciate for answer.