Learn Hardcore Dutch: De kleedkamer waarin de acteurs zich omkleden is vrij klein. - The dressing room in which the actors change is rather small.

Waarin is the fused form of waar + in, literally “in which.” You need a relative pronoun plus the correct preposition when the verb or action in the clause requires it. Here omkleden (to change clothes) takes in, so you fuse them into waarin.

Why is the verb omkleden placed at the very end of the clause?

Because this is a subordinate (relative) clause in Dutch. In subordinate clauses, the finite verb (here omkleden) moves to the final position:
… waarin de acteurs zich omkleden …

What is the role of zich before omkleden?

Zich is the reflexive pronoun meaning “themselves.” Omkleden can be transitive (I dress someone) or reflexive (I change my own clothes). Here the actors change themselves, so you use zich omkleden.

Can I split waarin and say waar de acteurs zich in omkleden instead?

Yes.
De kleedkamer waar de acteurs zich in omkleden is vrij klein.
This is slightly less formal but perfectly correct. You just keep the verb at the end (omkleden) and insert in before it.

Why is kleedkamer written as one word and not two?

Dutch loves compounds: two or more words join to make one. Kleed (dressing) + kamer (room) = kleedkamer (dressing room). Always write compounds together in modern Dutch.

What does vrij mean in is vrij klein?

Here vrij is an adverb meaning “fairly,” “rather,” or “quite.” It softens the statement: “is quite small.”

Why do we say de kleedkamer and not het kleedkamer?

In Dutch, most one-syllable nouns and many compound nouns take de. Kamer (room) is a common-gender noun that uses de, so de kleedkamer.

Should there be a comma before waarin?

No. Dutch relative clauses introduced by waarin (or waar) are not set off by commas. You write:
De kleedkamer waarin de acteurs zich omkleden is vrij klein.