I understand making small machines is fun, and I did it once myself. I regretted it every day I used that machine, though and have gone back to "normal" sized cases.
I ask as its interesting to me how much less stuff goes into a PC these days. When I built my first box around 2000, it had so many components inside:
Motherboard/CPU/RAM Graphics Card
Sound Card Network Card TV Tuner Card Floppy Drive CD Burner drive DVD Drive (this prob came after initial build 2 3.5" HDs
That second portion of the list is effectively gone now, integrated into the motherboard, and M2 drives are the size of ram sticks and don't take up any significant space. Coolers have gotten bigger, as have high end graphics cards, but PC cases seem to generally be about the same size, when they are holding much less.
What did you regret about a small case?
The problem with SFF is airflow. If everything is crammed together you are going to have to pump it through with high pressure and small fans which means noise, lots of it, if you want to have a high performance rig.
If you don't need performance, just get an all-in-1 or remodel a laptop build into the back of your monitor.
Here's some builds that use it: https://pcpartpicker.com/builds/by_part/wBw7YJ
Airflow is bad in small cases. And it is harder to build it for newbie correctly.
Bigger cases are usually quieter (fans are bigger and spin less).
I would recommend MicroATX format and case. Still fairly small but without any compromises.