Hi,
Unfortunately CMS doesn't allow to supply key size as an attribute for CSR.
You need to create your own CSR outside of CMS MMP with whatever attributes you want. For example, you could use a Windows domain-joined workstation to create a CSR, issue the certificate with exportable private key, then split the public cert and the private key into distinct files. Once that's done upload the two files to your CMS.
Take a look at Appendix A of this guide:
https://www.cisco.com/c/dam/en/us/td/docs/conferencing/ciscoMeetingServer/Deployment_Guide/Version-3-10/Certificate-Guidelines-for-all-Deployments-3-10.pdf
It's important that the files you upload to CMS aren't password protected, otherwise they key can't be matched to the certificate. Make sure to make user of "pki match <key> <cert>" to verify it works well.
