Αυτό το μπισκότο είναι πιο μεγάλο απ’ όσο νόμιζα, οπότε δεν θέλω άλλο γλυκό.

That is the normal Greek pattern.

Greek usually says:

• αυτό το μπισκότο = this cookie
• literally: this the cookie

So after a demonstrative like αυτός / αυτή / αυτό, Greek normally keeps the definite article before the noun.

The forms also have to match the noun:

• αυτό = neuter singular
• το = neuter singular article
• μπισκότο = neuter singular noun

So αυτό το μπισκότο is the expected, natural way to say this cookie.

Because Greek adjectives must agree with the noun in gender, number, and usually case.

Here both nouns are neuter singular:

• το μπισκότο = neuter singular
• γλυκό = neuter singular

So the words describing them are also neuter singular:

• μεγάλο = big
• άλλο = other / another / more

That is why you see the -ο ending.

A quick comparison:

• μεγάλος = masculine
• μεγάλη = feminine
• μεγάλο = neuter

And similarly:

• άλλος
• άλλη
• άλλο

Yes, basically.

Both mean bigger:

• πιο μεγάλο
• μεγαλύτερο

Greek has two common ways to form the comparative:

• πιο + adjective → πιο μεγάλο
• a one-word comparative form → μεγαλύτερο

In everyday Greek, both are natural.
For this sentence, Αυτό το μπισκότο είναι πιο μεγάλο... and Αυτό το μπισκότο είναι μεγαλύτερο... are both fine.

A small nuance:

• πιο + adjective is often felt as very straightforward and common in speech.
• the one-word comparative can sound a bit more compact or slightly more formal, depending on context.

This is the Greek way to say than I thought after a comparative.

So:

• πιο μεγάλο = bigger
• απ’ όσο νόμιζα = than I thought

The full form is από όσο, but before a vowel Greek often shortens από to απ’:

• από όσο
• απ’ όσο

You can think of the structure like this:

• πιο μεγάλο απ’ όσο νόμιζα
• bigger than I thought

The word όσο here is part of the comparative pattern. It does not mean just as much by itself in this sentence; it helps build the idea than I had thought / than I expected.

You will see the same pattern in many similar sentences:

• Είναι πιο δύσκολο απ’ όσο περίμενα.
• It is harder than I expected.

Νόμιζα is the imperfect of νομίζω, and it often describes an ongoing past belief, assumption, or expectation.

So here it means something like:

• than I had been thinking
• or more naturally in English, than I thought

Greek often prefers the imperfect for background mental states such as:

• believing
• thinking
• hoping
• knowing

Νόμισα is the aorist, which sounds more like a single completed moment of thinking:

• I thought
• it occurred to me
• I assumed

In many contexts both can be possible, but the nuance changes a bit.

Here, νόμιζα fits well because the speaker is comparing reality with a previous expectation.

Here οπότε means so, therefore, or as a result.

So the sentence moves like this:

• This cookie is bigger than I thought, so I don’t want any more dessert.

It introduces the consequence of the first part.

Be aware that οπότε can also have other uses in Greek depending on context, but in this sentence it is clearly a result word: so / therefore.

Very natural alternatives in some contexts could be:

• άρα
• λοιπόν
• γι’ αυτό

But οπότε is very common and natural here.

Because in Greek, δεν normally goes directly before the verb to negate it.

So:

• θέλω = I want
• δεν θέλω = I do not want

You do not place it after the verb the way English sometimes separates negation with do not.

This is the standard pattern:

• δεν ξέρω = I don’t know
• δεν μπορώ = I can’t
• δεν θέλω = I don’t want

Because άλλο γλυκό here means more dessert / any more dessert, not necessarily another separate dessert item.

So:

• δεν θέλω άλλο γλυκό = I don’t want any more dessert
• δεν θέλω ένα άλλο γλυκό = I don’t want another dessert / one more dessert

The version without ένα is more general and natural here. It treats γλυκό more like a category or an amount of dessert, not one specific countable piece.

So the sentence is not really about ordering a second dessert item. It is more like saying, I’ve had enough sweet stuff.

It can mean both:

• γλυκό as an adjective = sweet
• γλυκό as a noun = dessert / sweet thing

In this sentence, it is a noun.

So:

• άλλο γλυκό = more dessert / another sweet

This is very common in Greek.
For example:

• Θες γλυκό; = Do you want dessert?

So even though γλυκό literally relates to sweetness, in everyday Greek it very often means dessert.

Greek word order is fairly flexible, but the sentence you have is the most neutral and natural order.

Neutral order:

• Αυτό το μπισκότο είναι πιο μεγάλο απ’ όσο νόμιζα, οπότε δεν θέλω άλλο γλυκό.

You can move elements for emphasis, but then the tone changes.

For example:

• Άλλο γλυκό δεν θέλω.
  This emphasizes I don’t want any more dessert.

• Πιο μεγάλο απ’ όσο νόμιζα είναι αυτό το μπισκότο.
  This is possible, but more marked and less neutral.

So yes, Greek can rearrange things, but the original version is the safest standard one for learners.