Rekli su mi da će sljedeća rata biti veća jer je kamata porasla.

Croatian very often leaves out subject pronouns when the verb already makes the subject clear.

In Rekli su mi:

• su marks 3rd person plural
• rekli also shows a plural subject

So Croatian does not need oni. Adding oni is possible, but it gives extra emphasis or contrast:

• Rekli su mi = neutral
• Oni su mi rekli = they told me, as opposed to someone else

Rekli is the past active participle of reći and is part of the Croatian perfect tense.

The perfect is built with:

• a form of biti as an auxiliary: su
• the participle: rekli

So rekli su means they said / they told.

The ending -li shows masculine plural or a mixed-gender plural group. If the speakers were all female, it would be:

• Rekle su mi

Because reći uses the dative for the person who receives the information.

So:

• mi = to me
• me = me as a direct object

Croatian says literally:

• Rekli su mi = They said to me / told me

This is why mi is correct here.

In this kind of sentence, da is normally required.

After verbs like reći, znati, misliti, čuti, Croatian usually uses da to introduce the content clause:

• Rekli su mi da... = They told me that...

English often drops that, but Croatian usually does not drop da here.

This is the normal way to make the future in Croatian.

Future I is formed with:

• the clitic auxiliary će
• the infinitive biti

So:

• će biti = will be

That gives:

• da će sljedeća rata biti veća = that the next installment will be bigger

So the future is not in sljedeća rata itself; it is expressed by će biti.

Će is a clitic, and clitics usually appear very early in the clause.

After da, it is very natural to say:

• da će sljedeća rata biti veća

As for biti će, that is not the standard form here. Standard Croatian uses:

• će biti

You may also see forms like bit će, where the infinitive loses its final -i when the auxiliary comes after it. But biti će is not the standard choice.

Because they both agree with rata, which is a feminine singular noun.

So here:

• sljedeća rata = feminine singular adjective + feminine singular noun
• veća also agrees with rata

This is normal Croatian adjective agreement:

• sljedeći kredit = masculine
• sljedeća rata = feminine
• sljedeće pismo = neuter

And similarly:

• veći
• veća
• veće

The adjective velik has an irregular comparative.

Its comparative is:

• veći = bigger, greater

Then it changes for gender and case:

• veći = masculine singular nominative
• veća = feminine singular nominative
• veće = neuter singular nominative

Since rata is feminine singular, you get:

• veća rata = a bigger installment

So veća is simply the feminine form of the comparative veći.

Yes. In this sentence, rata is a feminine noun meaning installment or payment installment.

It is a different word from rat meaning war.

So here:

• sljedeća rata = the next installment

This is a useful word in financial contexts, such as loans, bills, and mortgage payments.

Two things are happening here.

First, je is a clitic, so it often appears near the beginning of the clause:

• jer je kamata porasla = very natural
• jer kamata je porasla = possible, but more marked or emphatic

Second, porasla agrees with kamata, which is feminine singular.

The perfect tense here is:

• je porasla = has risen / rose

If the subject were masculine, you would get porastao.
If it were feminine, as here, you get porasla.

Because the sentence describes three different time relationships:

• Rekli su mi: the act of telling happened earlier, so it is in the past
• će ... biti veća: the next installment is still in the future
• je kamata porasla: the interest rate has already risen, and that is the reason for the future larger installment

So the timeline is:

1. the interest rose
2. they told me
3. the next installment will be larger

That mixture of tenses is completely natural.