Ĉi tiu porcio estas tro granda.

In Esperanto, ĉi tiu is the normal way to say this.

• tiu = that
• ĉi adds the sense of here / near the speaker
• so ĉi tiu literally works like that one here, meaning this

You can also say tiu ĉi, which means the same thing. Both are correct:

• Ĉi tiu porcio = this portion
• Tiu ĉi porcio = this portion

The difference is distance or emphasis:

• ĉi tiu = this one, near me
• tiu = that one, farther away or already identified

So:

• Ĉi tiu porcio = this portion
• Tiu porcio = that portion

Because porcio is the subject of the sentence, and subjects do not take -n.

In Esperanto:

• -o = noun
• -on = direct object

Here, the sentence structure is:

• Ĉi tiu porcio = the subject
• estas = is
• tro granda = too big

So porcio stays in the basic noun form, without -n.

Because granda is an adjective, and adjectives in Esperanto end in -a.

Examples:

• granda = big
• bona = good
• varma = warm

In the sentence, granda describes porcio, so it must be an adjective.

Because granda describes a noun (porcio), so it must be an adjective.

• granda = big, large (adjective)
• grande = in a big way / greatly (adverb)

Compare:

• La porcio estas granda. = The portion is big.
• Li manĝas grande. = He eats in a big way / extravagantly.
  (This is much less common and sounds quite different.)

So here, only granda fits.

Tro means too, in the sense of more than is good, suitable, or wanted.

So:

• granda = big
• tro granda = too big

This is like English too big, not just very big.

Compare:

• tre granda = very big
• tro granda = too big

That distinction is important.

They are not the same:

• tre = very
• tro = too much / excessively / more than is acceptable

Examples:

• La porcio estas tre granda. = The portion is very big.
• La porcio estas tro granda. = The portion is too big.

A learner should be careful here, because English speakers often confuse very and too when learning new languages.

Estas is the present tense of esti, meaning to be.

• esti = to be
• estas = is / am / are

Esperanto uses the same present-tense form for all persons:

• mi estas = I am
• vi estas = you are
• li estas = he is
• ĝi estas = it is

So Ĉi tiu porcio estas tro granda literally follows the same pattern as English:

This portion is too big.

Yes. In Esperanto, adjectives agree with the nouns they describe in number and case.

Here:

• porcio is singular
• it is not accusative
• so the adjective is also plain singular, non-accusative: granda

Compare:

• granda porcio = a big portion
• grandaj porcioj = big portions
• Mi vidas grandan porcion. = I see a big portion.

In your sentence, both forms stay simple:

• porcio
• granda

Yes, grammatically that is possible, but it changes the feel.

• Ĉi tiu porcio estas tro granda. = This portion is too big.
• Ĉi tiu granda porcio estas tro granda. = This big portion is too big.

The second version sounds a bit repetitive in English and Esperanto, because you are already calling it big and then saying it is too big. It is possible, but usually unnecessary.

Esperanto has the definite article la, but it is not always needed when English would naturally think of the.

Here, ĉi tiu already identifies the noun clearly:

• ĉi tiu porcio = this portion

Because this already points to a specific portion, adding la would be wrong. You do not say:

• la ĉi tiu porcio ✘

So ĉi tiu does the job by itself.

Yes, Esperanto word order is fairly flexible, but some orders are more natural than others.

The most natural version here is:

• Ĉi tiu porcio estas tro granda.

You may also see:

• Tiu ĉi porcio estas tro granda.

But changing other parts too much can sound marked or unnatural. For example, moving tro away from granda is usually not a good idea, because tro granda works as a unit.

So for learners, the safest pattern is:

subject + estas + tro + adjective

A rough English-friendly guide:

• Ĉi ≈ chee
• tiu ≈ TEE-oo
• porcio ≈ por-TSEE-o
• estas ≈ ES-tahs
• tro ≈ tro
• granda ≈ GRAHN-dah

A few useful points:

• ĉ sounds like English ch in church
• c sounds like ts
• stress in Esperanto usually falls on the second-to-last syllable

So:

• por-CI-o
• ES-tas
• GRAN-da

Yes, porcio is a normal and useful word for portion, especially of food.

Depending on context, Esperanto might also use other words, but porcio is straightforward and idiomatic here.

Examples:

• malgranda porcio = a small portion
• granda porcio = a large portion
• duobla porcio = a double portion

So in this sentence, porcio is exactly the kind of word you would expect.