Confusion in was and were: The diameter length of X,Y,Z was/were 4,5,6 respectively
I guess that the sentence is the answer to question. So it is impossible to be sure sure whether the answer quoted is to a question, whose wording we do not have. But I should have said "The diameters of the three plates, X, Y and Z, were 4,5 and 6, respectively". The word 'length' is superfluous: a diameter is a length. Better still, "The three plates X,Y and Z **had diameters of 4,5 and 6, respectively.