When can a point charge serve as a test charge?

What is a test charge ?

To measure electric field at a point due to a positive charge, positive test charge q_(0) is put at the point. The measured electric field is

In defining electric field due to a point charge, the test charge has to be vanishingly small. How this condition can be justified, when we know that charge less than the on an electron or a proton is not possible?

Potential at a point due to a point charge is V. The charge is doubled and also the distance of the point from the charge is doubled. The new potential is