Read file size from tape

I have a tape with a .tar.lzma archive written to it. How can I determine the size of the .tar.lzma archived _as is_? In other words, I'm looking to find the size of the compressed archive, _not_ the size of the uncompressed .tar it is storing -- ideally without having to copy the archive off the tape to a disk and checking from there. The tape in question (LTO-6) is **not** formatted with any special file system, such as LTFS. My system is using Debian 11 Bullseye on amd64, if it is relevant to mention this.