Although we don't suggest that you use 2048 key pair, but I would like to introduce how to generate 2048 key pair manually

Generate 2048 key pair manually:

First of all, we need to generate a certificate which contains public key/private key. We can use MakeCert.exe (.NET Framework Tools) to generate 2048 certificate like this:

makecert -pe -n "CN=mydomainkeys" -ss my -sr LocalMachine -a sha1 -sky signature -r -len 2048


Then you can check the certificate like this:
Windows Start Menu->input:
MMC

press enter.

MMC->File Menu->Add/Remove Span-in
Add->Choose "Certificates"-> Computer Account->Local Machine->Finish->Close.

Then you should find the certificate at
"Certificates(Local Computer)->Personal->Certificates"

Right click your certificate->Export->"Yes, export the private key"->input password->save it to *.pfx file.

Then you can use above 2048 certificate in DKIM manager.