The question is, how much space do i need to install OMV5 and have some left to play with "containers" (and for updates)?
To answer your original question: 16 would be big enough but a 32 allows for room to allow the Flash Memory Plugin to work and reduce write-to wear and extend the life of your system drive. A 32GB usb stick is cheap (get two; one for backup).
If you are going to set up on the new machine you’ve built, don’t worry so much about the state of your system drive on the old machine, as long as you can still use it till the new machine is ready to transfer data over.
Before installing docker set the docker file path in the omv-extras tab to /srv/“any data disk you like”/docker.
You are really better off with a fresh install of OS and containers on your new machine.