I concur with the man from the UK!
For me, 250Gb for Windows, 2Tb for MSFS w/ORBX, P3Dv5 w/ORBX, Assetto Corsa Competizione, and F1 2021. Still plenty of room for more add-ons.
I got a data center rated 10Tb HDD with the intention of saving Manual Cache for off-line use. But ASSobo/Microsoft lied about MSFS being usable without a internet connection.
There are now two speeds for NVME (I got the last generation).
PCIe Gen3 or PCIe Gen4
Be sure to get whatever speed your board supports.