Electronic – How are cpus enumerated/identified/counted by BIOS and other low-level routines

It's like candy bars. They keep making them smaller at the same price to increase profit.

Seriosly though, there are good reasons for smaller chips. The first and foremost is that more chips can be fit onto a wafer. For large chips, the cost is all about what fraction of a wafer it uses. The cost to process a wafer is pretty much fixed, regardless of how many chips result from it.

Using less of the expensive wafer is only one part though. Yield is the other. All wafers have imperfections. Think of them as being small but randomly scattered about the wafer, and any IC that hits one of these imperfections is trash. When the wafer is covered by lots of small ICs, only a small fraction of the total are trash. As the IC size goes up the fraction of them that hit a imperfection goes up. As a unreal example that nonetheless points out the issue, consider the case where every wafer has one imperfection and is covered by one IC. The yield would be 0. If it were covered by 100 ICs, the yield would be 99%.

There's a lot more to yield than this, and this is greatly oversimplifying the issue, but these two effects do push towards smaller chips being more economical.

For really simple ICs, the packaging and testing cost dominates. In those cases, the features size is not so much a driving issue. This is also one reason we have seen a explosion of smaller and cheaper packages lately. Note that extreme small features size is being pushed by very large ICs, like main processors and GPUs.

What you're thinking of doing could be accomplished with 74xx series medium scale integration (MSI) chips, such as were available in the late 1970's, and more easily with some of the more recent 74xx's that were added in the early 1980's. If you can find a databook for this series, especially a later, mid-1980's one, that has pin-outs and logic diagrams, it will be invaluable for such an endeavor.

If you can look at the specs for the 74LS374 and understand how it might serve as an 8-bit register, it shouldn't be too much of a stretch to see how you could connect these on buses so as to be able to move data around between them. Investigate the 74LS181 and you can basically get all the math capabilities of the venerable 8-bit 6502 CPU in one swoop. The trickiest part is the control circuitry to execute instructions by sequentially operating all the registers, buffers, and ALU in the right order. If you only took a couple of EE classes, this will probably be your major stumbling block.

To get oriented with this level of design, you might try something like "Microprogramming and Computer Architecture" by Bruce Segee and John Field. Used copies can be had for cheap at a certain on-line book seller.