Eu só quero uma fatia pequena, porque já comi bastante.

In Portuguese, só often means only / just, and it commonly goes before the verb it modifies.

So:

• Eu só quero... = I just want...
• literally: I only want...

This is the most natural placement here in Brazilian Portuguese.

You may also see other word orders in some contexts, but Eu só quero is the standard, everyday way to say it.

Yes. Portuguese often drops subject pronouns when the verb already makes the subject clear.

So both are natural:

• Eu só quero uma fatia pequena
• Só quero uma fatia pequena

Because quero clearly means I want, the pronoun eu is optional.

Including eu can add:

• emphasis
• contrast
• clarity in conversation

For example, Eu só quero uma fatia pequena can sound a little more personally emphatic, like Me, I just want a small slice.

In Portuguese, adjectives usually come after the noun.

So:

• uma fatia pequena = a small slice

This is the most neutral and common order.

You can sometimes put adjectives before the noun, but that often sounds more literary, expressive, or changes the nuance. For everyday speech, fatia pequena is the safest choice.

Fatia usually means slice — a piece that has been cut in a flat, slice-like way.

Examples:

• uma fatia de pizza = a slice of pizza
• uma fatia de bolo = a slice of cake

Pedaço is more general and means piece or chunk.

So:

• fatia = slice
• pedaço = piece

In this sentence, fatia suggests something normally served in slices, such as cake, pie, pizza, etc.

Because adjectives in Portuguese usually agree with the noun in gender and number.

Here, the noun is:

• fatia — feminine singular

So the adjective must also be feminine singular:

• pequena

Compare:

• uma fatia pequena = a small slice
• um pedaço pequeno = a small piece

So pequena matches fatia.

Because Portuguese does not always need an explicit that in this kind of sentence.

English:

• because I’ve already eaten a lot

Portuguese:

• porque já comi bastante

This is fully natural and complete in Portuguese. The connection is handled by porque = because.

This is a very common question.

In your sentence, porque is one word because it means because:

• ...porque já comi bastante. = ...because I’ve already eaten a lot.

Very roughly:

• porque = because
• por que = why / for what reason
• porquê = the reason
• por quê = why (usually at the end of a question)

Examples:

• Não fui porque estava cansado. = I didn’t go because I was tired.
• Por que você saiu? = Why did you leave?
• Não entendi o porquê. = I didn’t understand the reason.
• Você saiu por quê? = You left why?

Já usually means already.

So:

• já comi = I already ate / I’ve already eaten

It shows that the eating happened earlier than the current moment, and that earlier action explains why the speaker now wants only a small slice.

In conversation, já is extremely common and very useful.

Portuguese often uses the simple past where English may use either:

• I ate
• I have eaten

So:

• já comi bastante can mean:
  • I already ate a lot
  • I’ve already eaten a lot

In Brazilian Portuguese, comi is the normal, natural choice here.

A form like tenho comido exists, but it usually means something more like I have been eating or I’ve been eating repeatedly, not a simple completed action. So it would not fit as well here.

Here bastante means a lot or quite a lot.

So:

• já comi bastante = I’ve already eaten a lot

Bastante is very common and flexible. It can work as:

• an adverb: comi bastante = I ate a lot
• an adjective/determiner in some contexts: bastantes pessoas = many people

In this sentence, it works adverbially, modifying comi.

Yes, you could say:

• porque já comi muito

That is grammatical and natural.

The difference is small:

• muito = very common, direct, often simply a lot
• bastante = also common, sometimes feels like quite a lot / enough / plenty

In this sentence, both work well, but bastante can sound slightly more like I’ve had plenty already, which matches the idea of wanting only a small slice.

Both orders are possible.

Your sentence:

• Eu só quero uma fatia pequena, porque já comi bastante.

Also possible:

• Porque já comi bastante, eu só quero uma fatia pequena.

The version with porque second is more natural in everyday speech when you first state what you want and then explain why.

So the sentence structure is:

1. statement: Eu só quero uma fatia pequena
2. reason: porque já comi bastante

That is a very common pattern.

Very natural. In casual speech, a Brazilian speaker might reduce it a bit:

• Só quero uma fatia pequena porque já comi bastante.

In pronunciation, some common features may happen:

• só sounds like saw but shorter and more closed
• quero is roughly KEH-roo in Brazilian pronunciation
• pequena is roughly peh-KEH-na
• já sounds like zhah
• comi is roughly ko-MEE
• bastante is roughly bahs-TAHN-chee in many Brazilian accents

The sentence has a very normal conversational rhythm.

Yes. Both are good translations in this context.

• Eu só quero uma fatia pequena =
  • I just want a small slice
  • I only want a small slice

In natural English, just often sounds more conversational, while only can sound a little more precise or emphatic.

In Portuguese, só covers both ideas very naturally here.

It is neutral everyday Portuguese. It is not especially formal, but it is also not slangy.

You could use it:

• with friends
• with family
• in a restaurant
• at a party
• in most everyday situations

That makes it a very useful model sentence for learners.