For many years we had three Desktop PC sizes. We had the Tower, T, the Desktop, DT, and the Small Desktop, SFF. Then came the Ultra Small Desktop, USFF or USDT and the Micro Tower MT. And yes we had the IBM Full desktop aberation for too long. Of course not all manufacturers kept to the same acronyms just to make things easy, but that is another story, I am looking at you, DELL..
Now the newest form factor is the Desktop Mini, MFF, this is the novel/paperback book sized PC, and yes there are the small NUC from Intel and MacMini fom Apple, and HP had weenie aberration that seems to have died off. So as an industry standard the Mini MFF, as pictured is the smallest.