Mini-ITX (or just ITX, for short) is smaller than Micro- ATX.
ETA: NUC's aren't generally upgradeable, other than storage and memory. I'm not aware of any boards in that form factor that would allow you to drop in your own processor.
ETA 2: There's lots of ITX cases, from ones that can fit full size, 330mm GPUs to ones that can only fit the motherboard, low-profile CPU cooler, and an SFX PSU.
If I could make a recommendation, the NR200 Max is a great first case for SFF. I think it's about 17L, so on the larger end of SFF, but it can fit full size GPUs and a number of different CPU coolers.
SFF Time is a unique option that makes a couple cases that can fit ATX parts.
And if you're unsure, you can always ask the SFF Community.