Flexibility to use the most amount of hardware. So it would be all of the above, although running an EATX system in a tower requires an EATX supporting tower, so you're still committed one way or the other.
If this weren't your first actual server hardware rack build, then you might do something else. But that is where you will get into the chicken:egg problem. You can't afford to not go with a flexible configuration now, but later when you know beyond theory what you want to do, you *might* be able to pull off certain space-saver builds and make perfect sense out of it. In a home, rackspace is cheap, so I go with 4U-6U systems. Smallest items are power conditioners (1U) and switches (1U). I don't bother with anything smaller because it's simply easier to add some rack brackets somewhere else in the house and run another system because it takes quite a while until you start reaching capacity(to the point where it looks like clutter). In an office/warehouse/farm environment, the opposite is true.
12U is a pretty good starter. Just remember, the more shit is spinning and more circuits are buzzing, the louder that cabinet gets. One cool thing, for years I used a minicab from an early DEC PDP system. It was lime green on the outside with doors. Rack standards are 60+ years old, so you can find some awesome tech lore in used cabinetry. I still find purple Sun Micro cabs at dirt prices and in perfect condition from time to time sitting dead in warehouses, which are a bit more of a project, but just some food for thought if you're into vintage tech and restomodding(new guts in a classic shell).
Okay, let's try this method instead. Easy mode. Pick your motherboard, cpus, and cooling. From there see what case it fits in, PSU and you'll see exactly what you really should start with, size-wise.
How do you know what CPUs you need, well, calculate performance needs. Seems like a lot of work, but it isn't. Once you get an idea of what you want, it's easier to build what you need. SAME like a desktop, just more parts.
