El retrato de la mujer con sombrero estaba junto a una escultura muy grande.

Because retrato is a masculine noun in Spanish. The article has to match the noun’s grammatical gender, so it takes el.

A useful reminder: noun gender in Spanish is grammatical, not logical. A word can be masculine even if it refers to something connected with a woman. Here, retrato is masculine simply because that is the gender of the noun.

Here de means of in the sense of showing or depicting.

So el retrato de la mujer normally means the portrait of the woman = a portrait that shows the woman.

It does not usually mean the portrait belongs to her, although in another context de can sometimes show possession.

The most natural reading is that it describes la mujer.

So la mujer con sombrero means the woman wearing a hat or the woman with a hat.

In other words, the structure is understood like this:

el retrato
de [la mujer con sombrero]

Spanish often places descriptive phrases after the noun they modify, so con sombrero is attaching to la mujer here.

Spanish often omits the article after con when talking about clothing, accessories, or general characteristics.

So la mujer con sombrero sounds natural and means the woman with a hat / wearing a hat.

If you say la mujer con un sombrero, that is also possible, but it can sound a bit more specific, as if you are thinking of one particular hat rather than just describing her appearance.

Because this sentence is talking about location, and Spanish uses estar for location.

So:

• estaba junto a... = was next to...
• era would be used for identity, classification, or characteristics, not where something was located

For example:

• Era una escultura famosa = It was a famous sculpture
• Estaba junto a una escultura = It was next to a sculpture

Even if the location is long-lasting, Spanish still normally uses estar for physical position.

Estaba is the imperfect, and it is commonly used for background description or a state that is being presented as ongoing.

Here the sentence sounds like part of a scene description:

The portrait was next to a very large sculpture.

That is exactly the kind of situation where Spanish often uses the imperfect.

If you used estuvo, it would sound more like you are referring to a completed, bounded situation, often tied to a specific time period or event.

So:

• estaba = descriptive background, ongoing state
• estuvo = completed state, seen as a whole

Junto a is a fixed expression meaning next to, beside, or by.

The a is simply part of that expression. It is not the personal a and it is not optional here.

Examples:

• junto a la puerta = next to the door
• junto al coche = next to the car
• junto a una escultura = next to a sculpture

Notice that when a is followed by el, it contracts:

• junto a + el → junto al

But here there is no contraction because the next word is una, not el.

Because muy is used to modify adjectives and adverbs, while mucho is used mainly with nouns and verbs.

Here grande is an adjective, so you need muy:

• muy grande = very big

Compare:

• muy grande = very big
• mucha escultura = a lot of sculpture
• trabaja mucho = he/she works a lot

So mucho grande is not correct in standard Spanish.

In Spanish, adjectives often come after the noun, especially when they describe a basic quality such as size, color, shape, or nationality.

So:

• una escultura grande = a big sculpture
• una escultura muy grande = a very big sculpture

If an adjective comes before the noun, the meaning can sometimes become more subjective, literary, or emphatic.

Also, gran before a singular noun often means great in the sense of importance, not physical size:

• una gran escultura = a great sculpture / an impressive sculpture
• una escultura grande = a large sculpture

So in this sentence, muy grande after the noun clearly refers to physical size.