I created a vault on an external hard drive on an empty NTFS Partition (J:) with the size of 242 GiB. According to Windows, the resulting virtual drive and my C: Partition have identical size and free space (98 GiB overall size, 6 GiB free space). Inside the vault on Partition J:, I only can use those 6 GiB for data storage. Partition C: is the one where my Windows 7 and Cryptomator are installed.
The screenshot below shows the sizes of the three partitions as well as the Cryptomator app for verification of the name of the vault and the partition used.