just adding for future visitors:
this would be a correct db-config inside netdata.conf:
[db]
mode = dbengine
storage tiers = 3
dbengine tier 0 disk space MB = 256
dbengine tier 1 disk space MB = 256
dbengine tier 2 disk space MB = 256
No “multihost”!! In above context following would be wrong: dbengine tier 0 multihost disk space MB = 256
Answer question 1/2)
256 is the minimal size (a value below would still resolve to “256” in the web-interpretation in https://server01.xyz:19999/netdata.conf )
Answer question 3)
If the size of the “dbengine tier”-folder has grown to 700MB and one would configure the limit like dbengine tier 0 disk space MB = 256 and resetart with e.g. systemctl restart netdata the size of the respective dbengine-folder would directly shrink below that value