General synonyms:
* Inconceivable
* Unthinkable
* Unfathomable
* Improbable
* Beyond comprehension
* Beyond belief
* Beyond imagination
* Unforeseeable
* Astounding
* Mind-boggling
More specific synonyms:
* For vastness: immeasurable, boundless, infinite
* For horror: dreadful, terrifying, appalling
* For joy: ecstatic, blissful, euphoric
* For surprise: shocking, astonishing, startling
The best synonym will depend on the specific meaning you're trying to convey.