On the fusion drive, you cannot specify which data is kept on the SSD and which is migrated off to the spinning disk. This is up to the OS to do. However, you can pretty safely assume that your Lightroom catalog is going to be on the SSD along with the OS parts you use regularly.
How much SSD should be in the specs. For my 2012 Mac mini (last quad-core) , it's 128 GB. I really like it.