Ermelengden er riktig.

It is a compound noun:

• erme = sleeve
• lengde = length
• -n / -en = the definite ending, giving the sleeve length

So:

• ermelengde = sleeve length
• ermelengden = the sleeve length

Norwegian uses compounds very freely, so this kind of long noun is very normal.

Because Norwegian usually writes compound nouns as a single word.

So English sleeve length becomes Norwegian ermelengde.

This is very common:

• bokhylle = book shelf / bookshelf
• kaffekopp = coffee cup
• barnesko = children’s shoes

If you split compounds incorrectly in Norwegian, it can look unnatural or even change the meaning.

The ending -en marks definiteness. In Norwegian, the definite article is often attached to the end of the noun instead of being a separate word.

Compare:

• en ermelengde = a sleeve length (less natural in most real situations, but grammatically possible)
• ermelengden = the sleeve length

In this sentence, the speaker is probably talking about a specific garment, so the sleeve length is the natural idea.

Because that would sound more general or abstract, while ermelengden points to a specific sleeve length.

• Ermelengden er riktig = The sleeve length is correct/right
• Ermelengde er viktig = Sleeve length is important (generic statement)

So for a real item of clothing, the definite form is the normal choice.

Er is the present tense of å være = to be.

So:

• jeg er = I am
• du er = you are
• den er = it is
• ermelengden er = the sleeve length is

In this sentence, er simply links the subject to the description riktig.

Because riktig is a predicative adjective here: it comes after er and describes the subject.

In Norwegian, adjectives after er do not take the definite form just because the noun is definite.

So:

• den riktige ermelengden = the correct sleeve length
  
  • here the adjective is before the noun, so you get riktige
• ermelengden er riktig = the sleeve length is correct
  
  • here the adjective is after er, so riktig is correct

That difference is very important in Norwegian.

It is an adjective.

It describes the noun phrase ermelengden, but it does so through the verb er. This is called a predicative adjective.

So the structure is:

• Ermelengden = subject
• er = verb
• riktig = adjective describing the subject

It is not an adverb here.

Sometimes rett can also mean right/correct, but in this sentence riktig is the more natural choice.

Why?

Because riktig often means:

• correct
• appropriate
• proper
• the right one

With measurements, fit, size, or suitability, riktig is very common.

Rett can also mean things like:

• straight
• right/correct
• directly
• legally entitled

So Ermelengden er riktig sounds more natural for clothing fit.

Usually it refers to the sleeve-length measurement as one property of the garment, not literally just one sleeve.

Even though a shirt or jacket has two sleeves, you can still talk about the sleeve length as a single measurement.

If you wanted to talk about both sleeves more explicitly, you could say something like:

• Begge ermene har riktig lengde. = Both sleeves have the correct length.

But ermelengden er riktig is completely natural when talking about fit.

Yes. A few alternatives are:

• Lengden på ermene er riktig. = The length of the sleeves is correct.
• Ermene har riktig lengde. = The sleeves have the correct length.
• Ermelengden passer. = The sleeve length works / fits.

The original sentence, Ermelengden er riktig, is compact and very natural.

The sentence follows the normal Norwegian main-clause pattern:

• Ermelengden = subject
• er = verb
• riktig = complement

So it is simply:

subject + verb + description

Norwegian main clauses usually follow the V2 rule, which means the verb is in the second position. In this sentence, the subject comes first, so the verb comes right after it.

For example:

• Nå er ermelengden riktig. = Now the sleeve length is correct.

Here nå takes first position, and er still stays in second position.

A rough guide for an English speaker would be:

• ermelengden ≈ ER-meh-leng-den
• er ≈ air / ehr (depending on accent)
• riktig ≈ RIK-tih

A few helpful points:

• The r is usually clearly pronounced.
• The g in riktig is not pronounced like hard English g.
• The rhythm is smoother and lighter than English spelling might suggest.

If you want to sound natural, try saying it in three chunks:

ER-meh-leng-den | er | RIK-tih