Generate a PGP key with DSA key size of 1024 bits and El Gamal key size of 2048 bits using Mac GPG 2.0

Question

Can somebody please tell me how to generate a PGP key with Mac GPG 2.0 that uses a DSA key for signing with a length of 1024 bits, and an El Gamal key for encryption with a length of 2048 bits ? I've tried using the tool both through the GUI and the command line, and it seems it only lets you generate keys that have symmetric lengths (ie 1024 or 2048 for both). I need the one specified above to communicate with somebody using a similar key.