Does the Certification Authority have to use an official stamp in the relevant place when issuing a certificate?

There is no prescribed requirement for the size or style of ‘official stamp’. At an event the person inspecting the official stamp on a certificate has no means of knowing if it is the correct one or not. The important thing is that the issuing Certification Authority will recognise it at some future stage should the certificate be referred back to it for any reason.

Any size or style of official stamp will suffice providing the CA is happy that it can recognise it as authentic at a later stage.