DEC card form factor
The original small standard length FLIP CHIPs (used in the PDP-8 and KA10 PDP-10 processor) had a total board size of 4.93 inches (from the far end of the edge connector contacts, to the handle end of the board) by 2.437 inches (from one side to the other). From this basic size, a large array of other sizes was built up.
A board of the same width, but approximately twice as long (8.43 inches) was referred to an an extended length board; many later M-series FLIP CHIPs used this format. A board of the short length, but twice as wide (5.187 inches) was referred to as a double height board; boards of this size are not common, but do exist. (The 'height' does not refer to the length, but to the width; this is because these board were usually mounted vertically, with the side edges on the top and bottom.)
Extended length boards came in multiples of two, four (10.457 inches) and six times the width of the small original board; these are usually called dual, quad and hex height format boards.
Finally, as production processes improved, and leading signals from board to board became undesirable, even longer hex boards were introduced; these first appeared in the VAX-11/780, and were called super hex boards within DEC.
Edge connector contact identification
The edge connector contact groups (often called 'rows', in DEC documentation) on the edge of a hex card are 'numbered' A-F, starting from the top (when the card is installed, vertically); this would be the right-hand edge, when the card is horizontal, with the component side up, and the connector 'fingers' toward the viewer.
Within each row, the individual contacts are 'numbered' from the DEC Alphabet; those on the component side are further numbered '1', and those on the solder side, '2'. I.e. with the board in a horizontal position, as described above, the right-most contact on the top side (i.e. visible) would be identified as 'AA1'.