As n-gram length grows, we are still going to have the same number of ngrams, they go through a hashing function and indexed in the bloom filter as usual. The number of n-grams size n in a text is text_length - ngram_length + 1.
I tested this 6-gram "it won't find anything matching exactly", no match. Almost anything we write has never been said exactly like that before.
This approach is probably inadequate. In my line of (NLP) research I find many things have been said exactly many, many times over.
You can try this out yourself by grouping and counting strings using the many publically available Bigquery corpora for various substring lengths and offsets, e.g. [0-16]; [0-32]; [0-64] substring lengths at different offsets.