Security of ChaChaPoly (IETF rfc7539) in the noise protocol framework of the Lightning Network
The paper you cited proves that ChaChaPoly is IND-CCA secure. This notion of security seems to perfectly cover your attack if I read the definition right  (https://en.wikipedia.org/wiki/Ciphertext_indistinguishability): 
"A cryptosystem is considered secure in terms of indistinguishability if no adversary, given an encryption of a message randomly chosen from a two-element message space determined by the adversary, can identify the message choice with probability significantly better than that of random guessing (​1⁄2).". 

That is, even if just two such lengths existed, the adversary cannot guess which one is encrypted here significantly better than randomly.