Configuration of Volumio

I am new with “Volunio” and Raspberry. I would like help with the following topics:
How largeshould the SSD card be?
Does “Volumio” get along with “HiFiBerry Digi+ Standard”?
Can “Volumio” run headless?
How is the Raspberry then controlled?
Thanks a lot!
Christian

https://volumio.github.io/docs/User_Manual/Quick_Start_Guide