Un milione è un numero grande.

In Italian, milione is a noun, so it normally takes an article when you are talking about a million as a thing or concept.

• un milione = a million
• milione by itself would usually sound incomplete in this sentence

So Un milione è un numero grande literally starts with A million is..., just like English.

Because milione is a masculine singular noun beginning with a normal consonant, so the correct indefinite article is un.

Italian indefinite articles:

• un for most masculine singular nouns
• uno for masculine singular nouns beginning with s + consonant, z, ps, gn, and a few others
• una for feminine singular nouns

Examples:

• un milione
• un libro
• uno studente
• uno zio

So milione takes un, not uno.

It is masculine.

That is why you get:

• un milione
• il milione

And in the plural:

• i milioni

This also matters when adjectives agree with it in other contexts.

The accent distinguishes è (is) from e (and).

• è = is
• e = and

So:

• Un milione è un numero grande = A million is a large number
• pane e formaggio = bread and cheese

This accent is very important in writing.

È is the third-person singular form of the verb essere (to be).

So it means:

• I am = sono
• you are = sei
• he/she/it is = è

In this sentence, the subject is un milione, which is singular, so è is the correct form.

Because numero is also a singular countable noun, and in Italian it needs an article here, just as in English.

• un numero = a number

So the sentence structure is:

• Un milione = subject
• è = is
• un numero grande = a large number

Both milione and numero are singular masculine nouns, so both use un.

In Italian, adjectives often come after the noun.

So:

• un numero grande = a large number

This is the most normal order here.
Italian can sometimes place adjectives before the noun, but after-the-noun position is very common, especially in straightforward descriptive sentences.

Yes. Italian adjectives usually agree in gender and number with the noun they describe.

Here:

• numero is masculine singular
• grande is singular, and its singular form works for both masculine and feminine

Examples:

• un numero grande = masculine singular
• una casa grande = feminine singular
• numeri grandi = masculine plural
• case grandi = feminine plural

So in this sentence, grande is the correct agreeing form.

Both can relate to the idea of big, but they are not always used in the same way.

With something abstract like numero, Italian normally uses grande:

• un numero grande = a large number

Grosso is more often used for physical size, thickness, or bulk, though it can also have other meanings depending on context.

So here, grande is the natural choice.

Yes, that is possible, but it is slightly different in feel.

• un numero grande = a large number
• un gran numero = a great/large number

A useful detail:

• grande can shorten to gran before some singular nouns, especially in fixed or common expressions

However, un gran numero can also mean a large quantity of things in other contexts, as in:

• un gran numero di persone = a large number of people

So for a beginner, un numero grande is simpler and clearer in this sentence.

A simple pronunciation guide is:

oon mee-lee-OH-neh eh oon NOO-meh-roh GRAHN-deh

A few notes:

• gli does not appear here, so there is nothing especially tricky
• milione has the stress on -o-: mi-lió-ne
• numero has the stress on nu-: nú-me-ro
• grande has the stress on gran-

If you want a more Italian-like rhythm, try saying it smoothly as: un milióne è un número gránde

It is grammatically correct and perfectly understandable. It does sound a bit simple and explanatory, which is why it feels textbook-like, but it is still natural enough.

A native speaker might also say:

• Un milione è un numero molto grande.
• Un milione è già un numero grande.

But your original sentence is absolutely fine for learning basic structure.