What are the paraphrase parameters in openssl des3?

openssl des3" is really "openssl enc -des3". The password-based key derivation is a custom, undocumented scheme which, as far as password-based key derivation schemes go, is quite weak; see this answer (especially at the end) for some details. Basically, this is equivalent to hashing the password with a couple of MD5 invocations.

What matters for passwords is entropy, i.e. not the password length or the type of characters used; entropy is a measure of what the password could have been. See this answer for details on how to compute entropy. Remember that entropy is not computed on the password itself, but by analysing the process that generated the password. Since the key derivation scheme used by OpenSSL is quite bad, you need a lot of password entropy to achieve decent security. As a rule of thumb, the limit of computational power of serious attackers is 280 invocations of a cryptographic primitive like MD5 (rich attackers like the NSA would have much trouble reaching such computational levels; even very rich organisations like Google or Apple would find it quite challenging, and would not be able to do it discreetly). In this case, since trying a password means roughly computing two MD5, this means that the password entropy should exceed 279 -- i.e. "79 bits" because entropy (in cryptography) is normally expressed in bits (which is a logarithmic scale). IF you generate your passphrase as a sequence of random characters in a given alphabet, with each character being selected randomly, uniformly, and (crucially) independently of the other passphrase characters, THEN entropy can be easily computed: if you use n lowercase letters (26 choices for each character), then entropy is 26n. With n = 17 (17 random lowercase letters), then entropy is close to 280, i.e. 80 bits, which, as explained above, should be sufficient. Even a small character set can be compensated for by using a long-enough passphrase, because what really matters is the entropy, not the length or the kind of character.

If you try to generate passphrases that "make sense" in some way for a human, then entropy is a lot more difficult to compute, because human psychology is very bad at randomness. Again, see the answer on entropy computation.