Can you not just go in "server role" and get a drop down there?
Don't quote me on this, but on older CMs, I think there may have been circumstances where you could have gone to a larger size. Still, if you deployed 8.0 for a really small 1000 user or 2500 user system, just adding CPU/RAM to the virtual machine probably isn't enough. I almost want to say you can't because the OVA deployment prompts for the footprint, so I feel like there's some built-in restrictions once you deploy.
I'd take a full backup and try deploying a new vm somewhere with a different footprint size and restoring my small CM backup and see what happens.