What to use instead of "very small"?

Solution 1:

The idea of being 'very small' can be expressed by adjectives minute and infinitesimal.

The latter one is more appropriate in your case.

According to Merriam-Webster's Dictionary https://www.merriam-webster.com/dictionary/infinitesimal :

INFINITESIMAL

: immeasurably or incalculably small

// an infinitesimal difference

Solution 2:

You can say that a is minuscule:

minuscule adjective
\ ˈmi-nəs-ˌkyül\

2 : very small
// minuscule amounts
_{from m-w.com}

As you can see from the definition, however, "very small" is also a reasonable way to describe your thing.

(Note that it is often misspelled as "miniscule", probably on the presumption that it has mini- as a prefix driving the meaning of "small", but it is in fact derived from the Latin minuscula, "somewhat smaller".)