Longer disk-encryption passwords on Samsung Galaxy S5? I'm trying to encrypt my Samsung Galaxy S5 (AT&amp;T) and it only allows me to use a 16-character passphrase for my encryption key.

Is there a way to bypass this or change the encryption key to something longer? This is really annoying at best and is a bad security practice to limit the size of the passphrase.

(Android internally uses the Linux dm-crypt method for encrypting the disk, which doesn't mention such password-length restrictions, and I've used it in the past with very long passphrases.)
It is a poor password policy, however you should rest assured that a strong 16-digit password is sufficient to protect from brute force. As demonstrated by GRC, an exhaustive brute force attack would take 10 billion years using a massive cracking array: https://www.grc.com/haystack.htm