To report qtree sizes, we often just use a null quota... setting a null quota (using "-") allows you to view current usage with the "quota report" command. This is much faster than waiting for the output of du since the quota report size is updated realtime.
TR3425 gives quota examples...
# Quota Target type disk files thold sdisk sfile
To see the size of all qtrees in vol2 for example.. make an entry in /etc/quotas, then "quota on vol2" to enable quotas. There will be no enforcement of a quota just reporting of usage.
* tree@vol/vol2 - - - - -
If you want to hard set a limit then you could also do that using the default method above, or for individual qtrees a similar entry /vol/vol2/qtree1 tree@vol/vol2 - - - - -"