Well obviously it depends on the size of your offline music/video collection and amount of dockers you want to store but 256GB should be enough space IMHO. Computational power should be enough.
I'm really not that familiar with Docker on Mac because I've only used it briefly there but I believe the problem would be this.You need to do some really funky things for 256GB to not be enough diskspace for development and Docker (you'd want to keep containers as small as possible anyway).
Amazing. Thank you @dyn. I'm happily proven wrong.Storage of the containers, images, etc. will be on the local disk, not in the vm. The vm is only used to run Docker. You can actually change the disk image location from the Docker menu bar item (by default it is in ~/Library/Containers/com.docker.docker....).