Sizing for a flush countersunk screw
Wood screws like most other types of fasteners are made in standard sizes. You can find tables of data online that will show the size information that you are looking for. The image below shows one such table that should be able to guide you as to the screw head diameter for various common wood screws.



(Table Source: https://www.metalsheets.co.uk/wood-screw-guages-in-metric/)