Hi Eloise;
My book is on core construction but does touch on integrating it into a bell.
Bell size is determined by the internal surface area (ISA)
Each core size can utilize a certain bell size without adding a bypass.
Here are the published ISA numbers for specific core sizes.
From Peter's website
https://batchrocket.eu/en/building
Riser diameter / Internal Surface area
12.5 cm (5") ISA 3.7 m² (39.8 sq ft)
15.0 cm (6") ISA 5.3 m² (57 sq ft)
17.5 cm (7") ISA 7.2 m² (77.5 sq ft)
20.0 cm (8") ISA 9.4 m² (101 sq ft)
22.5 cm (9") ISA 11.4 m² (123 sq ft)
25.0 cm (10") ISA 14.7 m² (158 sq ft)
These isa numbers can be exceeded by installing a bypass to aid in starting a good draft.
EDIT) ISA is measured on the walls and the roof unless it is insulated.
The floor and the core are not measured.