Windows 10 Pro is quite happy with 20 gigs for itself. It's when you add stuff to it. So yes, 100 GB will be fine if you don't plan on adding much.
I've got mine on a dedicated 120 GB SSD.
And I've seen it work quite well on 20 GB partitions when everything else was on the server side (in corporations, you don't want data on the workstations, you want all your data on the server).