Learn Hardcore Croatian: Za zamjenu veličine trebam samo poslati kratku poruku. - For a size exchange I only need to send a short message.

Because the preposition za most commonly governs the accusative when it means for / in order to / for the purpose of.
So you get za + accusative:

za zamjenu (zamjena → accusative singular zamjenu)
Meaning: for an exchange / for exchanging (purpose).

What case is zamjenu and how do I know?

zamjenu is accusative singular of the feminine noun zamjena (exchange / replacement).
A quick clue is the ending -u, which is a very common accusative singular ending for feminine nouns ending in -a:

zamjena (nom.) → zamjenu (acc.)

Why is it zamjenu veličine and not zamjenu veličina or zamjena veličine?

Two things are happening:

1) zamjenu is accusative because of za (explained above).

2) veličine is genitive singular after another noun, showing what kind of exchange:

zamjena veličine = a size exchange / exchange of size

So veličine is genitive singular of veličina (size):

veličina (nom.) → veličine (gen.)

(veličina = “size” in general; veličine here means “of size”.)

Could I also say zamjenu veličina?

Yes, sometimes. zamjenu veličina would use genitive plural (veličina → veličina) and can sound like exchange of sizes (more general, potentially multiple sizes).
But zamjenu veličine (genitive singular) is very common for the standard “size exchange” idea.

How does trebam work here—why is there no “to” like in English?

Croatian uses trebati + infinitive to express need to:

trebam poslati = I need to send

There’s no separate word for English “to” before the infinitive in Croatian. The infinitive is a single word: poslati (to send).

Is trebam more like “I need” or “I should”? Could I use moram?

trebam usually means I need (to) / it’s necessary for me to, and it can be softer than moram.

trebam poslati = I need to send (often practical necessity)
moram poslati = I must send (stronger obligation)

In many everyday contexts they can overlap, but moram sounds more forceful.

Why is samo placed before poslati? Can it move?

samo (only / just) usually goes right before what it limits.

Here it limits the action:

trebam samo poslati... = I only need to send... (that’s the only required step)

You can also move it for emphasis, but meaning shifts slightly:

Samo trebam poslati kratku poruku. = emphasizes only (no more than that)
Trebam poslati samo kratku poruku. = emphasizes that the message is the only thing being sent (not a form, not a package, etc.)

Why is it kratku poruku and not kratka poruka?

Because poruku is the direct object of poslati (to send), so it’s in the accusative.

poruka (nom.) → poruku (acc.) And the adjective must match case/gender/number:
kratka (nom. fem. sg.) → kratku (acc. fem. sg.)

So: poslati kratku poruku = send a short message.

Is poruka specifically “text message” or any message?

poruka is a general word for message. In modern usage, poslati poruku very often implies a phone/message app message, and kratka poruka naturally fits that.

If you want to be explicit about SMS, you can say SMS poruka.

What’s the difference between zamjena and promjena here?

zamjena = exchange / replacement (swap one thing for another)
promjena = change (making something different)

For clothing sizes in a shop context, zamjena veličine is the natural choice: you’re exchanging one size for another.

How is this sentence typically pronounced (especially zamjenu, veličine, poruku)?

Key sounds for English speakers:

lj is a “soft L” (similar to the lli in million for many speakers): ve-li-či-ne doesn’t have lj, but you might see it elsewhere.
č = like ch in church → veli
- či
  - ne
ž = like the s in measure (not in this sentence, but common in Croatian)

Approximate guide (very rough):
Za zamjenu veličine trebam samo poslati kratku poruku.
= za zam-YEH-noo veh-LEE-chee-neh TREH-bam SAH-moh POS-lah-tee KRAHT-koo POH-roo-koo