Poszłam do sklepu, ale wróciłam z niczym, bo nigdzie nie było mojego rozmiaru.

Because the speaker is first person singular (I) and feminine.

In Polish past tense, the verb agrees with the gender of the person speaking:

• poszłam = I went (said by a woman)
• poszedłem = I went (said by a man)

And similarly:

• wróciłam = I came back / returned (woman)
• wróciłem = I came back / returned (man)

So this sentence tells us the speaker is female.

This is about aspect and also the difference between iść and pójść.

• iść / szłam often focuses on the process of going: I was going / I went (on foot, in progress)
• pójść / poszłam usually means a completed, single trip with a result or destination

In this sentence, Poszłam do sklepu means I went to the store as a completed action. That fits well because the sentence tells the whole story: I went there, but then I came back empty-handed.

So:

• Poszłam do sklepu = I went to the store
• Szłam do sklepu = I was on my way to the store / I was going to the store

After the preposition do, Polish normally uses the genitive case.

• sklep = store
• do sklepu = to the store

So:

• nominative: sklep
• genitive: sklepu

The preposition do often expresses movement toward a place:

• do domu = to home / home
• do szkoły = to school
• do sklepu = to the store

Because the expression is wrócić z czymś / z niczym.

The preposition z here means with, and after this z, Polish uses the instrumental case.

• nic = nothing
• instrumental form: niczym

So:

• wrócić z torbą = come back with a bag
• wrócić z zakupami = come back with shopping
• wrócić z niczym = come back with nothing / come back empty-handed

This is a very natural Polish expression.

Here it means with, not from.

Polish z can mean different things depending on context:

• z domu = from the house
• z mamą = with mom
• z niczym = with nothing

In wróciłam z niczym, the idea is I returned with nothing.

So even though wrócić already means to return, the phrase z niczym adds what the person returned with: nothing.

Because in Polish, multiple negatives are normal and required.

English says:

• There was my size nowhere ❌
• There wasn’t my size anywhere ✅

But Polish usually combines a negative word like nigdzie (nowhere) with a negative verb:

• nigdzie nie było = literally nowhere there was not

This is standard Polish grammar, not extra emphasis.

Other examples:

• Nigdy nie wiem. = I never know.
• Nikt nie przyszedł. = Nobody came.
• Nic nie mam. = I have nothing.

So nigdzie nie było mojego rozmiaru is exactly what Polish expects.

Było is the past tense of być (to be), in the neuter singular form.

In this sentence, nie było works like an impersonal expression meaning:

• there wasn’t
• there were not

So:

• nigdzie nie było mojego rozmiaru = my size wasn’t available anywhere

Polish often uses było / nie było in this kind of existence/availability sentence:

• Był problem. = There was a problem.
• Nie było czasu. = There was no time.
• Nie było mleka. = There was no milk.

Even if English uses there was / there were, Polish often just uses było / nie było.

Mojego rozmiaru is in the genitive case.

Breakdown:

• mój rozmiar = my size
• mojego rozmiaru = of my size / my size (genitive form)

This happens because nie było often takes what is missing or unavailable in the genitive:

• Nie było chleba. = There was no bread.
• Nie było miejsca. = There was no room.
• Nie było mojego rozmiaru. = My size wasn’t available.

Both words change because the adjective must agree with the noun:

• mój → mojego
• rozmiar → rozmiaru

No. That would be ungrammatical.

After nie było in this kind of meaning, Polish uses the genitive, so you need:

• nie było mojego rozmiaru ✅

not:

• nie było mój rozmiar ❌

This is one of the patterns learners need to memorize: nie ma / nie było + genitive

Examples:

• Nie ma kawy. = There is no coffee.
• Nie było czasu. = There was no time.
• Nie ma mojego rozmiaru. = They don’t have my size.

Because Polish usually expresses this idea as availability/existence, not as a direct subject sentence.

So instead of saying something like my size was nowhere, Polish naturally says:

• nigdzie nie było mojego rozmiaru
• literally: nowhere there wasn’t my size
• naturally: my size wasn’t available anywhere

This structure is very common in shops, restaurants, and everyday situations:

• Nie ma już biletów. = There are no tickets left.
• Nie było wolnych miejsc. = There were no free seats.
• Nie ma tego koloru. = They don’t have that color.

Bo means because and is very common in everyday speech.

So:

• ..., bo nigdzie nie było mojego rozmiaru. = ..., because my size wasn’t available anywhere.

You could also use ponieważ, which is a bit more formal or written:

• ..., ponieważ nigdzie nie było mojego rozmiaru.

Both are correct, but bo sounds more natural in casual conversation.

Yes, Polish word order is fairly flexible, though some orders sound more natural than others.

The original sentence:

• Poszłam do sklepu, ale wróciłam z niczym, bo nigdzie nie było mojego rozmiaru.

is very natural.

You could also say:

• Poszłam do sklepu, ale bo nigdzie nie było mojego rozmiaru, wróciłam z niczym.
  This is understandable, but less natural.

Or:

• Bo nigdzie nie było mojego rozmiaru, wróciłam z niczym.
  Possible, but now the because clause is emphasized.

Polish word order often changes for focus and emphasis, but the original version is the most neutral and idiomatic.

Yes, it is very close to an idiomatic expression.

Literally it means I came back with nothing, but in natural English it often corresponds to:

• I came back empty-handed
• I returned with nothing

Polish speakers use this phrase not only for shopping, but also more generally:

• Poszedł na rozmowę o pracę i wrócił z niczym. = He went to the job interview and came back with nothing.
• Szukaliśmy pomocy, ale wróciliśmy z niczym. = We looked for help, but came back empty-handed.

So it is both literal and idiomatic.

A man would say:

Poszedłem do sklepu, ale wróciłem z niczym, bo nigdzie nie było mojego rozmiaru.

The changed forms are:

• poszłam → poszedłem
• wróciłam → wróciłem

The rest of the sentence stays the same.

Not only, but that is the most likely meaning here.

Rozmiar means size in general:

• shoe size
• clothing size
• object dimensions

In this sentence, because of the context I went to the store but came back with nothing, mojego rozmiaru almost certainly means my clothing/shoe size.

So it is general vocabulary, but the situation makes the meaning specific.