Thx all. I run this …
PoolSize=$(midclt call pool.query '[[ "name", "in", ["DuffleBag"] ]]' | jq -r '.[].size')
… and it gives me the number of bites(?) in DuffleBag. Divide by 1024^4 and I get TiB … exactly the same as ‘zpool list’ but different from the usage dashboard.