Assume an attacker captures a piece of ciphertext encrypted with a symmetric key.
He knows all the details of the algorithm used to encrypt except for the size of the key used in this case.
Can he deduce the size of the key used to encrypt that particular piece of ciphertext?
According to this answer about the more general question of telling what algorithm is used, since ciphertext encrypted with a symmetric key looks like random, one cannot answer the question what key size it is using. https://crypto.stackexchange.com/a/138/15012