Full disclosure: I am one of the authors and the current maintainer of the eCryptfs userspace utilities.
Linux has a maximum filename length of 255 characters for most filesystems (including EXT4), and a maximum path of 4096 characters.
eCryptfs is a layered filesystem. It stacks on top of another filesystem such as EXT4, which is actually used to write data to the disk. eCryptfs always encrypts file contents, but it can optionally encrypt (obscure) filenames.
If filenames are not encrypted, then you can safely write filenames of up to 255 characters and encrypt their contents, as the filenames written to the lower filesystem will simply match. While an attacker would not be able to read the contents of
budget.xls, they would know what file names exist. That may (or may not) leak sensitive information depending on your use case.
If filenames are encrypted, things get a little more complicated. eCryptfs prepends a bit of data on the front of the encrypted filename, such that it can identify encrypted filenames definitively. Also, the encryption itself involves "padding" the filename.
For instance, I have an encrypted file,
~/.bashrc. This filename is encrypted using my key to:
Clearly, that 7 character filename now requires more than 7 characters to be encrypted. Empirically, we have found that character filenames longer than 143 characters start requiring >255 characters to encrypt. So we (as eCryptfs upstream developers) typically recommend you limit your filenames to ~140 characters.
Now, all that said, the Synology NAS is a commercial product that embeds and uses eCryptfs and Linux to encrypt and secure data on the device. We (the upstream developers of eCryptfs) have nothing to do with Synology or their products, though we're generally happy to see eCryptfs used in the wild. It seems to me that their recommendation of 45 characters is either a typographical error (from our 140 character recommendation), or simply a far more conservative estimate.