Learn Italian: Il bicchiere si è frantumato in pezzi così piccoli che non li trovo. - The glass shattered into pieces so small that I cannot find them.

Yes, you can say in pezzi talmente piccoli che... with no change in meaning. Both così ... che and talmente ... che are used to express a result, meaning "so [adjective] that [result]".

Why do we use the reflexive si è frantumato instead of just ha frantumato?

Frantumare is a transitive verb meaning "to shatter" or "to smash" something else. Because the glass is breaking on its own rather than breaking another object, we use the reflexive or intransitive form frantumarsi. Like all reflexive verbs in the past tense (passato prossimo), it takes essere, which is why we see si è frantumato.

Why does the sentence use in pezzi instead of a pezzi?

When describing a physical transformation or something breaking into smaller parts, Italian often uses the preposition in (like frantumarsi in mille pezzi or dividere in due). While a pezzi is also common, it is usually for states of being (like essere a pezzi, meaning to be exhausted or broken) or certain actions (like tagliare a pezzi, to cut to pieces).

There is a che in this sentence, so why is the verb trovo in the indicative mood rather than the subjunctive?