El pegat és petit.

El is the masculine singular definite article in Catalan, equivalent to the in English.

So:

• el pegat = the patch

Catalan normally uses articles in many of the same places as English, though not always in exactly the same way.

Pegat is a masculine singular noun in this sentence.

You can tell because it uses:

• el = masculine singular the
• petit = masculine singular form of small

If the noun were feminine, the article and adjective would usually change too.

És with an accent is the 3rd person singular of the verb ser:

• és = is

So:

• El pegat és petit. = The patch is small.

By contrast, es without an accent is usually a pronoun in Catalan, not the verb is.

The accent helps distinguish them clearly.

És comes from the verb ser, which means to be.

Here is the present tense of ser:

• jo soc / sóc = I am
• tu ets = you are
• ell / ella és = he / she is

In this sentence, el pegat is singular, so Catalan uses és.

In Catalan, adjectives often come after the noun.

So:

• el pegat petit = the small patch
• El pegat és petit = The patch is small

This is very normal in Catalan and many other Romance languages.

Because pegat is masculine singular, the adjective must agree with it.

Catalan adjectives usually agree in:

• gender: masculine or feminine
• number: singular or plural

So the forms are:

• petit = masculine singular
• petita = feminine singular
• petits = masculine plural
• petites = feminine plural

Since pegat is masculine singular, petit is the correct form.

Yes. They are related, but they are not the same structure.

• El pegat és petit = The patch is small.
  This is a full sentence with a verb.

• El petit pegat = The small patch
  This is just a noun phrase, not a complete sentence by itself.

So in your sentence, petit is part of the predicate after és.

A simple learner-friendly pronunciation guide would be:

• El ≈ el
• pegat ≈ puh-GAT
• és ≈ ess
• petit ≈ puh-TEET

Very roughly: el puh-GAT ess puh-TEET

A few helpful notes:

• The final t in petit is pronounced.
• The stressed syllable in pegat is the last one: -gat
• The stressed syllable in petit is also the last one: -tit

Pronunciation varies somewhat by dialect, but this is a good starting point.

Catalan can omit pronoun subjects when they are clear from the verb, but here the subject is a noun phrase, not a pronoun.

So:

• És petit can mean He/it is small if the context is clear.
• El pegat és petit explicitly states the subject: the patch

In other words, Catalan can drop pronouns, but not the actual noun if you want to mention it.

Yes. That would mean:

• Aquest pegat és petit = This patch is small

The difference is:

• El pegat = the patch
• Aquest pegat = this patch

Both are correct, but they do slightly different jobs.

In the plural, all the agreeing words change:

• El pegat és petit. = The patch is small.
• Els pegats són petits. = The patches are small.

Changes:

• el → els
• pegat → pegats
• és → són
• petit → petits

This is a good example of agreement across the sentence.

Here, ser is the natural choice.

• ser is commonly used for general characteristics or identifying qualities
• estar is more often used for states or conditions

Since small describes a characteristic of the patch, és petit is the normal form.

So:

• El pegat és petit = The patch is small

That is the standard way to say it.