La dependenta diu que la talla petita ja no hi és, però que en queda una de gran.

Dependenta means a female shop assistant / sales assistant. The masculine form is dependent.

So la dependenta tells you the person helping the customer is a woman. In everyday Catalan, this is a very normal word in a shop context.

Catalan often uses the definite article where English might not.

Here la talla petita means the small size: the specific size option being discussed. It is not just the general idea of smallness; it is a particular size in the set of available sizes.

So:

• talla petita = small size
• la talla petita = the small size

Because both parts depend on diu.

The structure is:

• diu que
  • first piece of information
• però que
  • second piece of information

So the sentence is organized like:

• she says that the small size is no longer available,
• but that there is still a large one left.

In English, we often omit the second that, but in Catalan repeating que is very natural and helps keep the structure clear and parallel.

Ja no means no longer or not anymore.

So:

• no hi és = it is not there
• ja no hi és = it is no longer there / it is not there anymore

The ja adds the idea of a change: it used to be available, but now it isn’t.

Hi is a very common Catalan pronoun that often refers to there, in that place, or more abstractly in that situation/context.

In ja no hi és, it is part of the expression ser-hi, which can mean to be there, to be present, or in this context to be available.

So la talla petita ja no hi és means something like:

• the small size is no longer there
• the small size is no longer available

English usually does not translate hi word for word here.

Here Catalan uses the expression ser-hi.

In a shop context, no hi és is a natural way to say that something is not there / not available. It sounds idiomatic for stock or availability.

Using estar here would not sound natural in the same way. So this is best learned as a common pattern:

• hi és = it is there / available
• no hi és = it is not there / not available

Because adjectives agree with the noun they describe, but not all adjectives change in the same way.

Talla is feminine singular, so:

• petit becomes petita

But gran has the same form for masculine and feminine singular:

• talla gran
• jersei gran

Only the plural changes:

• grans

So:

• talla petita is feminine agreement
• talla gran is also feminine, but gran already has that same singular form

En is a partitive pronoun. It often means something like of it, of them, or some.

Here it replaces an omitted noun phrase that is understood from the context: the item/model/garment being discussed.

So:

• en queda una = one remains of it / there is one left

This is very common with quantities in Catalan:

• Queden dues samarretes = Two T-shirts remain
• En queden dues = Two remain / There are two left

So en is used because the noun is not repeated.

Because quedar can mean to remain or to be left.

So en queda una literally means:

• one remains
• there is one left

This is a very common shop phrase, because it emphasizes the remaining stock, not just existence in a general sense.

So the nuance is:

• hi ha una = there is one
• en queda una = there is one left

Because when the noun is omitted and you want to describe it with an adjective, Catalan often uses de + adjective after words like un/una, numbers, and quantity expressions.

So:

• una de gran = a large one
• dues de petites = two small ones
• en vull una de blava = I want a blue one

That is the pattern being used here.

By contrast, una gran does not work the same way here. Also, gran before a noun often has the sense of great rather than big, so una gran... can suggest a different meaning.

In this sentence, una de gran means a large one or one in the large size.

Yes, you may hear that too, but the nuance is slightly different.

• ja no hi és focuses on the fact that the small size is not there / not available anymore
• ja no n'hi ha focuses more directly on there aren’t any left

Both are possible in shop language, but the sentence you have uses hi és for availability/presence and en queda una for what still remains. That pairing is very natural.