Hi Sgkjer,
Welcome to Affinity Forums :)
You didn't specify which program you're using.
There's no way to change the units to cm in the Export dialog but if you set up your document to cm in File ▸ Document Setup..., you can see the size of the objects in cm in the Transform panel on the bottom right of the interface (taking in account the DPI set in Document Setup) in Affinity Designer. You can also see the document size in cm in Document Setup dialog if you open it again after changing the units.
In Photo you can go to menu Document ▸ Resize Document and change the units to cm to check the size of the document in cm.
If this is something that is being sent to a professional printing company it may be best to contact them and ask which settings they prefer to have :)
