PCI card electromechanical specification
Yes there is a official specification, several versions in fact.

There is also a book about the PCI bus PCI Bus Demystified that is informative.

The specification is published by https://pcisig.com and can be ordered from them or you can search for it and ... https://www.ics.uci.edu/~harris/ics216/pci/PCI_22.pdf

PS. I used the search terms "PCI mechanical specification".