I looked and see that now there are 4 form factor cases
it's a little complicated :)
First, desktops are now divided into types: NUC (in fact a laptop as a small desktop box), desktop (notebook hardware + a monitor in one case) and old system blocks all different sizes. Themselves their sizes have not changed in 10 years, everything from ITX to E-ATX. But, since the iron has become less hot plus the SSD homes are increasingly becoming the only store, there is a tendency to reduce the size of the case. At least the rejection 3.5" basket for HDD already well reduces the body depth (ssd can be mounted above, below or to the side, i.e. right at all is the basket removed from the case).
So, again, if you take something that's not super - the total dissipation will be less than 10 years ago, then you will need a cooler smaller and less fans in the case.
Well, to choose iron - it is necessary to start to understand for what tasks, and what your budget is. And then see what fits what. If a lot of money and very powerful it is not necessary - and can be a very small computer to collect. If money is a little something cheaper to get a roomier case, most inexpensive, etc. Here is "in General" do not answer, specifics are needed.
PS right now there is a tendency of transition from 4хядерных to "dofiga-nuclear" processors with new AMD Ryzen percent. So right now to buy for the next 8-10 years, the idea is so-so, nemalovazhno today's 16ядерный top already after a couple of years is sketchy. Here is the past few years was stagnant, it was possible once to buy a i7-2600K, a few years of grief do not know, then another to disperse and voila - again top-end processor, nothing slows, and a few years of grief do not know.