How do you specify the key size of a Certificate Signing Request?
This should work for you:
openssl req -new -newkey rsa:2048 -keyout your.key -out your.csr
-newkey rsa:size - type and size of the private key
The way i prefer to do this is to edit the openssl.cfg and change the "default_bits" to "2048". In this way all keys you create will automatically start at the right size