The probability of breaking symmetric encryptions The probabilty of cracking AES is $1 \over 2^n$, n being the size of the key. Would this hold true for every encryption of the same size?

This is from RC4's Wikipedia page:


  In March 2015 researcher to Royal Holloway announced improvements to their attack, providing a $2^{26}$ attack against passwords encrypted with RC4, as used in TLS.


This might answer my question, but I'm not sure.
Technically the Caesar cipher is a symmetric encryption algorithm and it is broken so easily it barely requires pen &amp; paper.

So no, this does not hold.

Also, I'm not sure where you got that figure but that is the probability of guessing the correct key in one attempt. Given a modern computer it should at least be feasible to attempt two guesses. ;)