use of helping verb

There is no helping verb in sight here.

Let's breakdown your sentence. "Magnitude of words matter" --> (subject = singular noun) + (adjective phrase consisting of [preposition] + [plural noun]) + (verb = present tense, plural form).

In doing this, we can see the answer to your second question. Your subject does not agree in number with your verb.

The magnitude of words matters.

The plural form of magnitude is magnitudes. Also, you need the definite article "the" before magnitude.

I hope this helps.