I hope you have already tried using different browsers and are selecting the 2048 key length. Try Firefox or Chrome.
You can try restoring the certificate and you have an option to do that on ZD WebUI :: Configure :: Certificate :: Restore to Default Certificate/Private Key unless you have an older firmware on the ZD.
Otherwise, you can try from the CLI:
Ruckus (config)# certificate
Ruckus (config-certificate)# restore
Please keep in mind that ZD will restart after you apply the changes so kindly run these commands after production hours. After that you can login to ZD and request a CSR again.
This worked. The interface doesn't make it clear that you have to use the re-generate private key option before the CSR will be generated as 2048 length. As a feature request the key length should probably be 2048 by default as 1024 is antiquated.