The first prototype is when linked against libmcrypt 2.2.x, the second when linked against libmcrypt 2.4.x.
mcrypt_get_key_size() is used to get the size of a key of the specified cipher.
mcrypt_get_key_size() takes one or two arguments, the cipher and module, and returns the size in bytes.
See also: mcrypt_get_block_size().