Is it possible to determine the number of characters used for a password used to encrypt a file with AES 128? I lost a password for an ethereum wallet at the end of 2016. Computer immediately shutdown and password was not recoverable. 

I know there are many potential avenues for narrowing my search for what my password could have been, but I am wondering if the number of characters can be determined from the encrypted file?

I dont know if this would make a difference but it was an ethereum wallet, using Geth
No. Password cannot be used to encrypt anything directly unless it is exactly of a certain size, and then it will be likely weak. Encryption passwords are hashed, with a slow hash hopefully, and the output is used as key.