If ransomware encrypts your files, would it be possible to find the decryption key if you also have an unencrypted copy of a file that is as large or larger than the length of the decryption key, by comparing the files?

The decryption key for ransomware cannot typically be discovered by comparing an encrypted file with its unencrypted counterpart. Ransomware encryption mechanisms are usually designed to be strong and irreversible without the specific decryption key that was used to encrypt the files in the first place.