The kneidel should be minimally the size of a Talmudic egg (about twice the size of our average egg), but this is just a Hiddur.
In order to be yotzai you must of course consume a kzayis within 2-9 minutes depending on your shita.
The kazayis refers to an average density kneidel. If a Bubby who was born anywhere in Europe makes them, then due to their great hardness, a correspondingly lesser amount will suffice.