Det ene glas er for lille, så jeg tager det større.

Here ene means one in the sense of the one / one of the two.

So det ene glas means something like:

• the one glass
• one of the glasses
• often more naturally in English: one of the two glasses

This wording usually suggests a limited choice, very often two things. Danish often uses det ene ... det andet ... for the one ... the other ....

So in this sentence, det ene glas suggests the speaker is comparing one glass with another.

Because glas is a neuter noun in Danish: et glas.

That affects the form of words around it:

• et glas = a glass
• det glas = that glass / the glass in certain structures
• det ene glas = the one glass

You use det, not den, because den is for common gender nouns, while det is for neuter nouns.

Also, et ene glas would not work here, because this expression uses det ene as a set phrase meaning the one.

Because lille is irregular.

Many Danish adjectives add -t with a neuter singular noun:

• et stort glas
• glasset er stort

But lille does not become lillet. It stays lille:

• et lille glas
• glasset er lille

So Det ene glas er for lille is correct.

Here for means too.

So:

• for lille = too small
• for stor = too big
• for dyr = too expensive

This is a very common Danish pattern.

So Det ene glas er for lille means That one glass is too small or One of the glasses is too small.

Because større is the comparative form: bigger.

• stor = big
• større = bigger
• størst / største = biggest

In this sentence, the speaker is choosing between glasses and takes the bigger one, not necessarily the biggest one in some larger group.

So:

• det større = the bigger one
• det største = the biggest one

If there are two glasses, det større is exactly what you would expect.

Because Danish, like English, can leave out the noun when it is already understood.

So det større is short for:

• det større glas = the bigger glass

Since glas has already been mentioned, repeating it is unnecessary.

English does the same:

• I’ll take the bigger one instead of
• I’ll take the bigger glass

So det større is very natural Danish.

Not quite.

In det ene glas, det is part of the expression modifying the noun glas.

In det større, det acts more like a pronoun meaning the one / that one, with the noun left out.

So they look the same, but their roles are slightly different:

• det ene glas = the one glass
• det større = the bigger one

A learner can think of the second det as standing in for glass.

Because så here is being used as a conjunction meaning so.

It joins two main clauses:

• Det ene glas er for lille
• så jeg tager det større

After a coordinating conjunction like this, Danish keeps normal main-clause word order:

• jeg tager

If så were used more like then at the beginning of a clause, you could get inversion:

• Så tager jeg det større = Then I’ll take the bigger one

So both patterns exist, but they mean/use så a little differently.

Usually it strongly suggests two, or at least a very small, clearly defined set.

Very often Danish uses:

• det ene = the one
• det andet = the other

So in this sentence, most people will imagine two glasses, where one is too small and the other is bigger.

It does not have to be mathematically exact in every context, but that is the normal interpretation.

Yes. That is completely correct.

• jeg tager det større = I take the bigger one
• jeg tager det større glas = I take the bigger glass

The version without glas is more compact because the noun is already understood. The version with glas is more explicit.

Both are natural; the shorter version is especially common in everyday speech.