Punkteringen er liten.

Because Norwegian usually shows the by adding an ending to the noun instead of using a separate word.

• punktering = base form
• en punktering = a puncture
• punkteringen = the puncture

Here, -en is the definite singular ending.

In Bokmål, punktering is normally treated as a common-gender noun, so you use:

• en punktering
• punkteringen

That matters because adjectives in Norwegian change form to match gender and number.

Because liten is the common-gender singular form of the adjective.

The main forms are:

• liten = common gender singular
• lite = neuter singular
• små = plural

Since punkteringen is singular and common gender, liten is the correct form.

Because lille is usually used when the adjective comes before a definite noun:

• den lille punkteringen

But in Punkteringen er liten, the adjective comes after er. That makes it a predicative adjective, and Norwegian uses liten, not lille, in that position.

So:

• den lille punkteringen = adjective before the noun
• punkteringen er liten = adjective after er

Er is the present tense of å være, meaning to be.

So this is the Norwegian equivalent of is.

A useful thing to remember is that Norwegian uses the same present-tense form for all persons:

• jeg er
• du er
• han/hun er
• vi er

No. This is a very basic sentence pattern:

• Punkteringen = subject
• er = verb
• liten = adjective/complement

So the structure is essentially:

subject + verb + adjective

That works much like English in sentences such as The car is big or The hole is small.

You may hear forms like that in some dialects, in Nynorsk, and in Bokmål that uses more feminine forms.

That version uses:

• punkteringa instead of punkteringen
• lita instead of liten

But Punkteringen er liten is a standard, safe Bokmål form, and it is the best default for most learners.

Yes. It is the normal word for a puncture or flat tire, especially when talking about tires.

A very common expression is:

• å få punktering = to get a flat

So this sentence sounds natural if someone is talking about the size of the puncture.