Grinders break beans into pieces, then break the pieces into smaller pieces, and so on. Like breaking a piece of toast or a cookie; when you break a bean, you get the pieces -- called grinds -- and the crumbs -- called fines. The ideal grind has pieces all the same size (so they extract at the same rate) and just the right number of fines to suit the brewing process -- no fines for steeping like French press, some fines for pourover, a lot of fines for higher pressure processes like vac pots or espresso.
That's about all we know for sure.
We used to think bigger grinders with bigger burrs were better since they had a longer grind pathways that broke the beans more gently. Moreover, a bigger grinder built to the same tolerances as a smaller one would have smaller relative errors. On the whole, this has been a fairly accurate rule of thumb. The 68mm - 71mm conical burr and the 64mm flat burr have been highly successful designs that have been used for decades. The 68mm conicals outperform the 64mm flats, which in turn outperform the smaller burrs found in most home, semi-pro, and commercial "decaf" grinders. That has been the status quo.
Recently, a new player, Mahlkoenig, a builder of industrial and lab grinders, has been moving into the cafe and home fields, building burrs for Baratza and under its own name. These have been smaller than the Italian burrs, and have performed well above expectation for their size. In addition, very light third wave roasts are not roasted long enough for the beans to get into their "glass phase," therefore, these beans are less brittle and pose grinding challenges for conventional grinders. It has become fashionable to use Turkish coffee grinders for these roasts. These two factors have posed some challenges to the conventional wisdom about grinder quality.