Clay shrinks at every stage of the making process so you may need to be able to calculate the final size.
You can find information about the shrinkage of a particular clay by visiting the clay supplier’s website – sometimes it’s even on the pack. Once you have the shrinkage rate, use the calculators here:
https://www.omnicalculator.com/everyday-life/clay-shrinkage
You could also use rulers that account for shrinkage when making. You can find a selection of rulers here:
https://www.printable-ruler.net/pottery-shrinkage/