The external section size of a finished kitchen flue is mainly related to the number of storeys, the higher the number of storeys, the larger the size. The finished pipe wall thickness is only about 1cm, so you don't need to consider the internal dimensions.

Commonly used duct external section dimensions:

Below 6 storeys: 320X240

Below 12 storeys: 340X300

Below 18 storeys: 430X300

Below 24 storeys: 460X400

Below 30th floor: 600X400

Below 36th floor: 600X500

Because the pipeline will be installed after the main body is completed, the floor slab should be reserved for slab holes