encryption program question

Hi All,

  I have written an encryption program as a fun little side project and I
think I remember hearing something years ago about the length of an
encryption key having to be a prime number of digits long in order to be
considered even moderatly strong, but I can't find anything about that
requirement online. Are there any encryption experts here that know whether
or not that is true? I know that prime numbers are used in PKE (public key
encryption) systems and RSA seems to make use of them in the generation of
the encryption keys, but the use of the prime numbers doesn't seem to be
connected to the length of the encryption key itself from what I can tell.
Any takers for this one?  Thanks guys.