How to change CSR key size?

Hi,

I want to generate a CSR for my server.  I have used STRUST to do this. When I sent generated CSR to One of the CA providers, Its giving me the below error:

• Your CSR contains a key size that is no longer considered secure. Security best practices require a minimum key size of 2048 bits. Please submit a new CSR with a minimum 2048 bit key size.

Can somebody tell me how to change the key size?