How is original plain text length reconstructed after decryption under RSA?
Most importantly, note that for almost all practical applications textbook RSA should not be used to encrypt something. It does not yield an encryption scheme which provides the guarantees one would expect in practice. In particular it does not even provide security against chosen plaintext attacks (IND-CPA).

Instead one would use some padding scheme such as optimal asymmetric encryption padding (OAEP).

Despite the fact that the usage of RSA you describe should not be used in practice, you would typically not pad "XXXXXXXXXXXX" as "XXXXXXXXXXXX000000....". Instead, you would encode "XXXXXXXXXXXX" as a number between 1 and N and use this upon encryption (i.e., you only implicitly prefix it with zeroes).