You can either ignore it (you will see that it always report the C: drive capacity, regardless of how much data you add) or you can switch from WebDAV to Dokany. When you configure Cryptomator to use the latter, this issue should be solved.
In this case, you can not ignore what Windows Explorer tells you. You have to use Dokany, then both problems should be resolved.
Dokany is included in the Cryptomator installer. If you didn’t uncheck it, you should be able to see the option to switch to Dokany from Cryptomator’s settings. Otherwise just re-run the installer.