It is estimated that even the fastest computer would take millions of years to decrypt the file.
Textbook example for a non sequitur.
The file size has absolutely nothing to do with how long it would take to break the encryption code. A 1kB file encrypted with the same key would take exactly as long to decrypt.
If the file is too short it can not be decrypted as there would be several possible solutions. In order to have a definite solution the corpus should have a certain minimum length.