Generation of 1024 bits key certificate signing request-sun one app srvr 7

Kindly help to generate 1024 bits key certificate signing request in sun one application server 7. The problem faced by me is that during csr generation the key lengh is 512 as this is the default value.Now i would like to change this default value and would like to generate a key with length 1024.Kindly help me.

Thanks in advance

Vishnu Priyan