This is very dependent on how many tracks are in your library and what OS / machine you plan on using.
I highly recommend you plan to install twice. Install the first time in the most convenient way possible. If that’s on a Windows or Mac machine plugged into a USB DAC do that. Then take some time to familiarize yourself with the interface and options. Then you’ve got some solid footing on exactly what your permanent / always on Roon install will be going forward.
But, to answer your question… I’m using ~3GB for a lib of 16k tracks. That does not include the music files themselves.