Sadly, Windows cannot handle files larger than 4 GiB via WebDAV. Since the virtual drive is currently based on WebDAV, no files larger than that can be encrypted using Windows Explorer.
That said, Cryptomator can easily handle much larger files if the vault is accessed using some better software, such as Cyberduck. Here is a guide on how to do that: Can I use alternative WebDAV clients?
We’re trying to work around this limitation by providing the virtual drive using some other technology than WebDAV in future versions. The progress is documented on GitHub.