What is the usable area (for connectors) of a PCI Express card bracket?
According to the PCIe Card Electromechanical Specifications (CEM), we have the following specification for connector openings:



Image Source - CEM Spec rev 1.1, Section 6, Figure 6-2, page 73

So your magic numbers are 89.9mm x 12.06mm.

That diagram also shows you how the space positions relative to the PCB.

If you follow that spec and it doesn't fit into a given case, it is not your problem as you've conformed to the specification that the case designer should also have conformed to.