The PC/104 standard only defines one board size, which is common for board format specifications.
As shown in the graphic, the board is 3.550 by 3.775 inches with a board height of 0.6 inches.
Most board standards specify one form factor for commonality, but PC104 also does this because these boards need to stack atop each other.
In addition the boards need to conform to the maximum size of a case [if used] which is also specified.

There is one Printed Wiring Board [PWB] size [dimension] defined in PC/104.
However the board may be produced in three different configurations; the standard form is with either a PC XT bus or PC AT bus.
With the addition of the PCI bus, the specification is called PC/104 Plus. Up to six cards may be stacked together.
The board or Circuit Card Assembly [CCA] mechanical dimensions are shown above.
An additional standard; PCI-104 removes the IBM XT and AT buses, and only uses the PCI bus [J3], but board form factor remains the same size.
Boards may be stacked 6 high when using the PC-AT [PC/104] bus, but only four of the boards may use the PCI bus.

Back to the main PC/104 bus page for a description, connector manufacturers, IC manufacturers and links..

The PC/104 standard may be one of the few instances when there's no harm in still using the PC/XT or PC/AT interface.
After all these modules only communicate with one another over those connectors, and with the out-side world using another connector.
So a PC/104 board using the PX-XT connector [top graphic] is not constrained by an obsolete mother-board, but only requires itself.
Maybe another PC-104 card if they wanted to communicate over that old interface structure.
Of course the down side would be finding a chip-set that still operates using the PC-AT protocol.

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