When using a utility to encrypt data, you are not directly using the password you enter as the encryption key. Instead, the key is passed through a KDF, or Key Derivation Function, which converts it into the appropriate length. This allows you to use a password smaller or larger than the cipher's key size. In the case of OpenSSL, a single iteration of either MD5 or SHA-256 (depending on version) is used.
A very simple example of a KDF (although not one considered secure for these purposes) would be the SHA-256 function applied directly on the input. Your one-letter password is hashed into a 256-bit value which can then be used directly as the key. The same thing happens if your password is longer.
SHA-256("a") = ca978112ca1bbdcafac231b39a23dc4da786eff8147c4e72b9807785afee48bb
Note that the OpenSSL command line utility is not secure, as it is designed for testing the library's functions. It is not to be used for general purpose encryption. The biggest issue is that the KDF it uses is not slow (it uses only one hash iteration, whereas a secure one like PBKDF2 can use tens of thousands or more), which makes brute force and dictionary attacks far more efficient. However, it has other problems such as the lack of integrity. You should use a tool designed specifically for secure encryption, such as GnuPG (which uses a secure KDF called S2K) with its symmetric encryption functionality.