What factors are limiting form factor of e-ink displays?
EInk displays use standard TFT processes for the backplane and are fabricated in standard TFT foundries.  A standard LC (Liquid crystal) based display will have the front glass added and be filled with LC where as an EInk display has the micro-capsules added and the front film added.  EInk was very deliberate to ensure that they did not re-invent the wheel to get into production.  Their business model is to sell the front part of the display stack and NOT to manufacture backplanes.

How the panels is driven changes, but this is controlled by the column and row drivers which are relatively easily changed.

That means that the limitation in size is simply one of cost (since you could put a EInk stack onto any existing TFT back plane).  The cost is increased either because of direct cost charged by EInk or from poor yield in applying the films to a larger panel (poor yield increases cost, and increases more the later in the process).