Geological Differentiation: variables and size requirement
Not really.

Assuming even minor igneous bodies differentiate (stocks), and that differentiation is basically a process of decantation of the solid magmatic phase (a process known as fractional crystalization) and fractionation of the fluid magmatic phase, virtually any solid mixture can differentiate. If you're not convinced, here's another argument.

Let's take an analogous situation to differentiation: exsolution. They're not the same. Exsolution involves ionic migration in a solid structure. Differentiation involves ionic migration in a fluid (magma), which lacks structure. But both involve an outcome of different chemical phases. And exsolution can happen in microscopic sizes of milimetric crystals. So its fluid-ambient counterpart would, as a logical conclusion, be also able to happen on such minute conditions, and perhaps on smaller even.

And I'm not sure what you mean that differentiation will never end? On a long term geological scale - I'm talking about hundreds of millions to billions of years - that idea is perhaps correct, assuming the upper mantle and crust are constantly changing their compositions. But if you're saying that differentiation is a process that happens widespread (the entire crust, or the entire planet), rather than on specific locations (where partial melting occurs), under a small timescale (millions of years), then you're not correct, because differentiation only occurs in mixtured fluids (and I'm not sure whether it's used for any other fluid than magma).