Størrelseslappen viser størrelsen.

Because Norwegian usually writes compound nouns as one word.

So størrelseslappen is made up of:

• størrelse = size
• lapp = label, tag, slip, patch
• -en = the definite ending

Together, størrelseslappen means the size label/tag.

This is very normal in Norwegian. Where English often uses two words, Norwegian often makes a compound:

• skolebok = school book
• bussjåfør = bus driver
• størrelseslapp = size tag

That -s- is a linking s used in many Norwegian compounds.

So the structure is:

• størrelse
  • s
    • lapp
      • en

The -s- does not have a separate meaning by itself here. It just helps link the first part of the compound to the second part in a natural way.

You cannot always predict perfectly when a linking -s- will appear, so this is something learners gradually get used to through exposure.

Because they are doing different jobs in the sentence.

• størrelseslappen = the size tag/label
• størrelsen = the size

The first word is a compound noun: a label connected with size. The second word is just the noun størrelse in its definite form: the size.

So the sentence is structurally:

• The size tag shows the size.

Even though the same root word appears twice, that is perfectly normal.

In Norwegian, the is often added to the end of the noun instead of being a separate word.

Examples:

• en lapp = a label/tag

• lappen = the label/tag

• en størrelse = a size
• størrelsen = the size

So in your sentence:

• størrelseslappen = the size tag
• størrelsen = the size

This is one of the biggest differences from English noun structure.

The dictionary forms are:

• en lapp = a tag, label, slip
• en størrelse = a size

From those, you get the definite forms:

• lappen = the tag/label
• størrelsen = the size

And from the compound:

• en størrelseslapp = a size tag
• størrelseslappen = the size tag

Viser is the present tense of å vise, which means to show.

So:

• å vise = to show
• viser = shows / is showing

In Norwegian, the present tense is the same for all subjects:

• jeg viser = I show
• du viser = you show
• han/hun viser = he/she shows
• vi viser = we show

So Størrelseslappen viser størrelsen literally follows the same pattern as English:

• subject + verb + object

No, this sentence has very basic word order.

It follows the standard pattern:

• Størrelseslappen = subject
• viser = verb
• størrelsen = object

So it is simply:

• The size tag
  • shows
    • the size

This is the most neutral word order in a Norwegian main clause.

Yes, in this context it can.

The word lapp has a broad meaning and can refer to things like a:

• label
• tag
• slip
• patch
• little piece of paper or fabric

In clothing contexts, størrelseslapp is a natural way to refer to the little label/tag that shows the size.

Depending on context, English might translate it as:

• size tag
• size label
• size tab

The hardest part for many English speakers is ø.

A rough guide:

• ø is a front rounded vowel that English does not really have
• it is somewhat like saying eh or ur while rounding your lips

Very approximate pronunciation help:

• størrelse ≈ STUR-rel-seh or STER-rel-seh
• størrelseslappen ≈ STUR-rel-ses-lap-en

That is only approximate, but it may help you get started.

Also note:

• rr in Norwegian is not like English r
• the exact pronunciation varies somewhat by dialect

If you want to sound natural, it is best to listen to native audio and repeat it.

Because -n is added to make it definite.

The noun is:

• en størrelse = a size

The definite singular is:

• størrelsen = the size

This happens with many common Norwegian nouns:

• en bil → bilen
• en stol → stolen
• en størrelse → størrelsen

So størrelsen is just the regular definite form of størrelse.

Yes, but it changes the meaning slightly.

• Størrelseslappen viser størrelsen = The size tag shows the size
• En størrelseslapp viser størrelsen = A size tag shows the size

The first one sounds like you are talking about a specific tag. The second one sounds more general.

So both are possible, but they are not exactly the same.

It is understandable and grammatically fine. The repetition of the størrelse root is not wrong.

That said, in real-life speech, Norwegian speakers might also choose other wording depending on context, for example if it is obvious they are talking about clothing labels.

But as a learning sentence, it is very useful because it clearly shows:

• a compound noun
• a present-tense verb
• a definite direct object

So even if it feels a little repetitive to an English speaker, it is still a good and natural enough example sentence.