* For size: immense or énorme are the most common translations.
* For importance or impact: important or conséquent are more appropriate.
Here are some examples:
* An immense building: Un immeuble immense
* An enormous crowd: Une foule énorme
* A massive impact: Un impact important
* A massive victory: Une victoire conséquente
It's also possible to use massif directly, but it's less common and may sound more formal or technical.