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?

When comparing an encrypted file with its unencrypted counterpart, it is very unlikely to discover the decryption key for ransomware through this method. Ransomware encryption techniques are designed to be secure and nearly impossible to break without the specific key used to encrypt the files. Decrypting files encrypted by ransomware typically requires either obtaining the decryption key from the attacker (which is not recommended) or using tools developed specifically to decrypt files affected by known ransomware variants.