Comment by samwillis

According to a quick google search there are about 367million domains names registered. The max length is 253. So:

  367000000 * 253 bytes = 92.8GB

As you only need to know the domain exists.

In reality it would be smaller than that as the average length will be much shorted. I expect 10-20GB

You could get clever and use a bloom filter to save space, falling back to the api.