3rdKey for safety
First, don't think in terms of character length, but in terms of number of words.
Fairly short (5-10 character) words, easy to remember, seemingly random to anyone other than yourself.
Ideally you want five or six of them.
You'll use two or three for the first key, two or three for the second key, and one for the 3rdKey.
3rdKey.com will ask you for three characters from your 3rdKey, so a word that's easy to spell and easy to identify character positions. We ask for 3 characters within your 3rdKey at log in.
These need to be random words that no-one would associate with you, no matter how well they know you, but they can have some meaning for you. e.g. Very few people ever tell anyone who they had for a first crush, that person's name(s) would not be associated with you, but would seem random.
Better yet, use five or six entirely random words,
Generate Random Words
Choose one of the words for your 3rdKey, the one where we'll ask for three characters within the word. Then split the other four or five words into two separate keys, they'll be your first and second keys.
Your keys must be easy to remember
They need to be memorable, and you will not be able to recover them if you forget, so we'll ask for a reminder for each key, to help you remember in future.
But why use random words?
I have my five or six words, know what I'm going to use for each key, and am ready to register.