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. It may be possible to impose a weaker condition than equality on the lengths of the plaintexts, but the usefulness of doing so is not evident since there has to be some condition. For example, a condition that they should not differ by more than some prescribed value would just make things more complicated than they already are.
For more on this, see the exercises in the chapter about encryption schemes in the book of Goldreich.