Levels of strength: The Caesar Cipher SITREP: I am deciding what form of "Random" Number generation would fit best for a Caesar Cipher that encrypts each letter with a different integer as a shift. This encryption is being handled via C# code I have and will write. I have a few options for this as follows.  

(Note: The transmission of the key to the receiver is NOT a concern at this time and when it is a problem I will open a new question, this also includes how to hide which method I use to get the key, as that is a question for another time. Also to note, all random numbers not specified to be generated in a specific measure should be assumed to be generated from the C# code Random.Next(); Thank You.)

OPTIONS: 


User Entered. This can be up to the length of my message or below it in which case I can either use random numbers or simply repeat the sequence provided by the user (Please advise best option if this is the selected method).
Randomly Generated Numbers. I am aware of the use of OTP's but would also like to know how viable shorter keys are.
Computer or User Generated Keys. This method entertains many ideas such as a key linked to file size, time stamps, others. Is there another well documented use of encryption that serves well while being covert as to my use of it at the same time that I may look into?


ANSWERS: Please back up claims if they are not openly supported by the community here. If you would like to link to another question or article that may help please feel free to do so. Thank you for your time.
To be honest, I think all of them do NOT make much difference, since Caesar Cipher is a weak cipher. You can see Breaking the cipher on Wikipedia. 
It uses a method called frequency analysis, which ignores key strength.

Also, the three methods you mentioned all have their weaknesses. You should use a secure random generator.