Most Microsoft Windows installations plus Windows updates use somewhere around 20GB to 22GB give or take a couple of gigs.
Normally there is no reason to partition an ssd. Their capacity is simply not large enough to warrant it. In addition, there is no reason to manually format the ssd for use. Let Microsoft Windows do it automatically for you during the Windows installation process. One large C: drive. No muss! No fuss! No bother!
As for the free space just remember to check available ssd capacity every once in a while to make sure you do not fill the ssd to capacity. Keeping about 20% unused is a good general rule of thumb.