As far as the feature request goes, the submits I found in Ubuntu to implement guided_size (i.e. limiting the space in the VG that gets used) were: https://bazaar.launchpad.net/~ubuntu-branches/ubuntu/trusty/partman-auto-lvm/trusty/revision/19 https://bazaar.launchpad.net/~ubuntu-branches/ubuntu/trusty/partman-auto-lvm/trusty/revision/21
However, I do consider it a bug that (a) maximum size is exceeded and (b) the last partition/LV gets all remaining space, irrespective of priorities.