Security of Pohlig-Hellman exponentation cipher?
You need a large random prime modulus where the discrete log is hard.  Read about how to choose a prime so the discrete log is hard.  Also, you want $p-1$ to have as few small factors as possible.  Therefore, the short version is, I suggest you choose a large random 2048-bit prime $p$ such that $(p-1)/2$ is prime.

However, Pohlig-Hellman has some serious issues.  It has very poor performance, compared to other symmetric-key ciphers.  Also, it has no randomization, so it is not semantically-secure (it is not IND-CPA secure) in its standard form.  I don't know of anyone who uses the Pohlig-Hellman cipher in practice, in its standard form; it is more of theoretical interest than practical interest.

What exactly do you want to do with the Pohlig-Hellman cipher?  What exactly is your application?  Why do you want to use the Pohlig-Hellman cipher?  There might be better solutions.