This short paper presents preliminary work towards developing a variable-length symmetric-key cryptosystem. Introduced are the definition of a new class of encryption mechanism termed Polysizemic Encryption and a reference implementation named the “Slinky Cipher”. Three sub-classes of polysizemic ciphers are defined and examples of each are given. What makes a cipher polysizemic is discussed along with challenges associated with their evaluation. The Slinky Cipher is evaluated for its avalanche effect, and using the NIST statistical test suite. Preliminary baseline results show promising trends in terms of achieving provable security bounds, acceptable randomness ranges, and avalanche effect.