Learn Spanish (Spain): Mira, el castillo es inmenso, no necesitas exagerar. - Look, the castle is immense, you don't need to exaggerate.

You could combine the two halves by saying El castillo es tan inmenso que no necesitas exagerar (The castle is so immense that you don't need to exaggerate). This turns two separate statements into a single cause-and-effect sentence, exactly like the consecutive clauses you are practicing now.

Why use es inmenso instead of está inmenso here?

We use es (from the verb ser) because the size of the castle is a permanent, defining physical characteristic. If you used está, it would sound like the castle somehow magically grew and is only temporarily huge!

Is Mira being used as a command to physically look at the castle?

It can be, but it is often just used as a conversation filler to get someone's attention or introduce a point. It works exactly like saying 'Look, you don't need to exaggerate' or 'Listen, the castle is huge' in English.

Could I say no hace falta exagerar instead of no necesitas exagerar?

Yes! No hace falta is a very common phrase in Spain that means 'there is no need' or 'it is not necessary.' Saying no hace falta exagerar sounds very natural and means exactly the same thing.