Your password (the string of character you input on your computer) can be as long (within reason) or short as you want but generally passwords should be at least 8-10 alphanumeric characters, and should not be anything that can be guessed with enough information (eg, birthday, famous date, person's name).
We do not enforce any minimum password/PIN length on BC Vault! You can even choose to use no global/wallet password/PIN at all, but in that case, your data will not be protected well against 3rd parties! Always be sure to at minimum use a password or PIN (global or wallet)!
Also read: How long should my password/PIN be?, Password hints, How can I verify my wallet password/pin?