The photographs I found seem to be evenly divided between showing it as a compact unit that looks maybe one foot high
It is most likely, based on sales numbers, that you're describing a PDP-8/e or PDP-8/I. What you're seeing is just the front panel. The PDP-8/e front panel is 10.5 inches high (PDF!), or 6 rack units. The PDP-8/I front panel is a bit shorter, 5U high, or 8.75 inches. Another way to come to these values is by inference from the fact that DEC used standard 19 inch wide racks, that being the width of a panel mounted across the front of the rack. You can get an accurate aspect ratio from an undistorted photograph, which lets you determine panel height.
The front panel on the earliest PDP-8s was a separate unit, only a few inches deep, since it's only the switches, indicators, enclosure, and direct support electronics.¹ Indeed, some PDP-8s offered a choice of different front panels, such as a version with very few switches and indicators for embedded applications.
Early rack-mounted PDP-8s were made of many separate components, interconnected with ribbon cables and wiring harnesses, so the layout within the rack cabinet was flexible. The front panel was connected to the CPU proper and the power supply in this way.
Let's get concrete and focus on the PDP-8/I, the model I'm most familiar with.² Here's a dimension diagram from the PDP-8/I Maintenance Manual, volume 1, page 2-2:
The "console" in the diagram's upper left corner is what I've been calling the "front panel," and it's often all that's shown when you search for a picture of a PDP-8/I, even though it's really only one small-ish part of the whole computer. It's the sexy bit.³
As you can see in the diagram, some of the space left behind the front panel is taken up by the top end of the CPU proper, which was roughly 2 feet high by two feet deep and took up about half the rack width. The other half of that space was taken by the power supply.⁴ You can see that layout in these photographs. The diagram doesn't show the core memory as a separate component, since it was mounted within the processor chassis.⁵
Over their two decade span of development (1965-1985) PDP-8s moved from discrete transistors to ICs, then to denser and denser ICs, so they became smaller and more power-efficient. By the time of the PDP-8/e (1970), the CPU, core memory, power supply, and expansion slots were entirely behind the front panel. Its close relatives the PDP-8/f and 8/m used shallower chassis, but were the same basic idea.⁶ These systems were only self-contained within that 6U space if your application didn't require any sort of durable storage or I/O peripherals; add that in, and you're still talking about taking over a large portion of a full-size rack, at least.
The front panel would usually be mounted about halfway up the rack, since that's a convenient height for a seated person, who would normally sit at a Teletype Model 33 ASR, the standard terminal for DEC computers of the time. Since this is an essential component in many PDP-8 systems, it should also be considered part of the size of the computer: it's roughly the same width and depth as a rack cabinet, and about half the height. (Source)
Whereas the CPU proper is normally mounted below the front panel on the PDP-8/I and behind it on the PDP-8/e, it was mounted above the front panel on the original PDP-8. The original PDP-8 front panel + CPU + core + power supply assembly was about the same size as the PDP-8/I discussed above.
The rack cabinets usually used for PDP-8s were 5 or 6 feet high. The rest of the space in the CPU rack would be used for peripherals: TU56 DECtape drives, RK05 removable platter hard disks, etc.
It was not uncommon for a PDP-8 computer of the era to require a second rack for more peripherals: A/D conversion hardware, paper tape punch/reader units, more tape and disk storage, etc.
Eventually, the PDP-8 escaped the rack with the DECmate series, which packed a single-IC PDP-8 CPU, memory and a dual floppy drive into a large video terminal cabinet, not unlike early consumer-focused microcomputers.
The front panel electronics consisted of simple circuits like lamp drivers so that the power to drive the lamp didn't have to come from the CPU, only the low-current signal to turn it on.
I'm the current maintainer of the PiDP-8/I software project.
That's one reason the PiDP-8/I DIY project is basically just the front panel. Cloning the other parts hidden away by the rack isn't of general interest.
According to the same page in the manual where I found the above rack layout diagram, the power supply for a PDP-8/I is rated to provide about 1.7 kW of power, but another source says it only provides 380 W. I assume the latter is a measurement of steady-state current draw, and that the computer could surge into the kW range in some conditions. Other sources I've read say that a complete PDP-8 computer would draw roughly 1 kW, depending on the type and number of peripherals.
You may be reading this on a computer with a several hundred watt power supply which is much smaller than all of this and may be wondering at the discrepancy. There are many factors:
These early PDP-8 power supplies are inefficient linear-regulated power supplies rather than the switching-regulated power supplies that became universal for computers after this early PDP-8 era. They throw off more heat per unit power delivered to the point of load, so they need bigger heat sinks and more space for cooling air to circulate.
Modern power supplies are much more densely-packed than the DEC 704A.
These PDP-8 power supplies relied more on big transformers and big capacitors to provide clean DC output power than on clever electronics.
They were designed with discrete transistors rather than integrated circuit regulators, which were brand new technology at the time. IC regulators weren't capable of delivering the loads required by such computers.
In the PDP-8/I, the first field (4 kWords) of core memory is mounted close to the CPU proper because the timing of the CPU was based on the speed of core memory access. Indeed, the early PDP-8 CPUs weren't rated in MHz, but in core memory cycle times, with instruction times given as the number of core cycles they took. Memory reference instructions took more processing time than purely in-CPU operations. Core memory on a PDP-8 was like L1 cache in a modern CPU: tightly integrated and the fundamental limit on CPU processing speed.
The PDP-8/I then allowed a second 4 kWord field of core to be mounted on the CPU backplane along with a memory controller, the assembly being called the MC8I. This would then control rack-mounted MM8I core memory modules, each adding another 4 kWord field, for a maximum of 8 fields total, or 32 kWords.
The above details differ across the 20-year commercial history of the PDP-8. For example, the later PDP-8/a had the MS8C option which provided 16 kWords of solid-state static RAM in the form of an expansion bus card. The PDP-8/a was also the first to provide a way to get past the 32 kWord memory limit of all earlier PDP-8 models.
The PDP-8/f and 8/m were the first PDP-8s based on a switching power supply, rather than the linear-regulated supply of the PDP-8/e and earlier.