Find Uncompressed/Compressed Sizes of TimescaleDB Database/Table
If you want to find out savings from TimescaleDB's native compression, use this information view for the hypertable: https://docs.timescale.com/latest/api#timescaledb_information-compressed_chunk_stats

In the field, we're typically seeing seeing 90-98% storing savings across a large numbers of users.

For measuring ZFS, look at the numbers returned by zfs get compression and zfs get compressratio (see, e.g., https://www.servethehome.com/the-case-for-using-zfs-compression/)

If you are running native compression on top, you'll see that this is the amount of compression that ZFS adds on top of TimescaleDB's native compression.  Would love to hear what you see; would expect that to actually be pretty minimal (or even negative).