W restauracji porcja ryżu była tak duża, że nie zjadłem wszystkiego.

W restauracji means in the restaurant.

Here, the preposition w means in, and when it refers to location, it normally takes the locative case.
So:

• restauracja = restaurant
• w restauracji = in the restaurant

This is a very common pattern:

• w domu = in the house
• w szkole = at school / in school
• w sklepie = in the shop

Because ryżu is in the genitive case after porcja.

In Polish, nouns like porcja, kawałek, szklanka, kilogram often behave like a portion of, a piece of, a glass of, a kilogram of, so the second noun usually goes into the genitive:

• porcja ryżu = a portion of rice
• szklanka wody = a glass of water
• kawałek chleba = a piece of bread

So:

• porcja = portion
• ryż = rice
• ryżu = of rice

Because the subject is porcja, and porcja is a feminine singular noun.

In the past tense, Polish verbs agree with the subject in gender and number:

• był = masculine singular
• była = feminine singular
• było = neuter singular
• byli / były = plural

Since porcja is feminine, you get:

• porcja była duża = the portion was large

Even though ryż is masculine, it does not control the verb here. The main subject is porcja, not ryż.

Because after być in a sentence like this, the adjective usually matches the subject in the nominative.

So:

• porcja była duża = the portion was large

Here, duża agrees with porcja:

• feminine
• singular
• nominative

Dużą would be a different case form, and it would not fit this structure.

A useful contrast:

• To była duża porcja. = That was a large portion.
• Ona była dużą porcją. = It was a large portion.
  This second type is different and less common in everyday use here.

It means so large that...

This is a very common Polish pattern:

• tak + adjective/adverb + że... = so ... that ...

Examples:

• Było tak zimno, że nie wyszedłem. = It was so cold that I didn’t go out.
• Mówił tak szybko, że nic nie zrozumiałem. = He spoke so fast that I understood nothing.

So in your sentence:

• była tak duża, że... = was so large that...

Because Polish often leaves out subject pronouns when they are already clear from the verb form.

The ending in zjadłem already tells you it means I ate and that the speaker is masculine.

So:

• zjadłem = I ate
• nie zjadłem = I didn’t eat

Adding ja is possible, but it usually adds emphasis or contrast:

• Ja nie zjadłem wszystkiego. = I didn’t eat all of it.
  This can sound like I didn’t, maybe someone else did.

In neutral sentences, Polish usually omits ja.

Because zjadłem is perfective, and it focuses on the action as a completed whole.

• jeść = to eat, imperfective
• zjeść = to eat up / eat completely, perfective

In this sentence, the idea is about whether the speaker finished the food or not. That is why perfective works well:

• nie zjadłem wszystkiego = I didn’t eat all of it / I failed to finish it all

If you said nie jadłem wszystkiego, it would sound different. It would not focus as clearly on the result of finishing. The perfective form is the natural choice here.

Because zjadłem is the form used by a male speaker.

In Polish past tense, first-person singular forms show gender:

• zjadłem = I ate, said by a man
• zjadłam = I ate, said by a woman

So if a woman were saying the sentence, it would be:

• W restauracji porcja ryżu była tak duża, że nie zjadłam wszystkiego.

Because after a negated verb, Polish often uses the genitive instead of the accusative for the direct object.

Compare:

• Zjadłem wszystko. = I ate everything.
• Nie zjadłem wszystkiego. = I didn’t eat everything / I didn’t eat it all.

So:

• wszystko = accusative / nominative form
• wszystkiego = genitive form

This is a very important Polish pattern, especially with negation:

• Mam czas. = I have time.
• Nie mam czasu. = I don’t have time.

Not really in the broad English sense of everything in the world. In this sentence it means all of it or the whole thing, referring to the portion of rice.

So the idea is:

• the portion was so big
• I didn’t manage to eat all of it

That is why a natural English translation is often I didn’t eat it all, even though the Polish word is wszystkiego.

Polish word order is fairly flexible, but the version you have is natural and clear.

Starting with W restauracji sets the scene first:

• W restauracji porcja ryżu była tak duża, że nie zjadłem wszystkiego.

You could rearrange parts of the sentence, but the emphasis would change. For example:

• Porcja ryżu była w restauracji tak duża, że nie zjadłem wszystkiego.

This sounds less natural in most contexts, because w restauracji usually works best as background information at the beginning.

So the original order is a good, idiomatic choice.