No, the experiment does not remain valid because the ciphertext must leak *some* information about the length  of the plaintext, so if the lengths of the two plaintexts in the indistinguishability experiment are not restricted, the adversary could choose plaintexts of suitable, different lengths and use the information leaked by the ciphertext to gain an advantage.

For more on this, see the exercises in the chapter about encryption schemes in the book of Goldreich.