You can actually calculate the ciphertext size to the last byte:
- Each encrypted file gets a 88 bytes file header, see: https://docs.cryptomator.org/en/1.5/security/architecture/#file-header-encryption
- Every 32 KiB of data, 48 bytes will be added (nonce + MAC), see: https://docs.cryptomator.org/en/1.5/security/architecture/#file-content-encryption
But to make it simpler, let’s put the file header aside. You can roughly expect a 0,146484375% increase.
A 100 GB file cleartext file will then be roughly a 100.15 GB ciphertext file.