The standard PCI Express card form factor is 106mm x 311mm, as shown below.
The PCIe standard defines two sizes; a long form and short form factor board, with both having the same height [106mm].
Both PCIe card formats are shown below, note the keep-out for the short-form card in the center of the graphic.
Also note the component keep-outs shown in yellow at the front and rear of the boards.
The key location is also shown indicating the gap between the copper fingers at 249mm.
Unlike other PC board standards, PCIe requires very few pins and used a much smaller connector.

PCI Express Board Dimensions

At the time the PCIe spec was released these were the possible board sizes
It could be possible that other sizes are possible, but unlikely.
As with any board standard, component height off the board is normally also specified but not indicated here.
The size and shape of the PCIe board is also called the form factor of the card, and is different between expansion board standards.

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