These board are called flip chips because some early R and S series boards used a diode resistor hybrid with flip chip bonding of the diode. This did not work well and almost all boards I have seen use discrete components. Flip chip is now a production process used on CPU's and other large chips, DEC was a little too early.
The boards are 2.5x5 inches with originally 18 contacts on edge fingers on one side or double width with two sets of edge fingers., Later boards had 36 contacts on each edge connector using both sides and came in multiples of the width and height. Information on the creation of the Flip Chip modules. Posting on flip chips and another