Official comment
You set storage quotas in groups, and then assign users to the group with the amount of quota desired. If you don't already have a group with the size the user needs, you must first create a new group with that quota size, and assign the user to the group. When creating the new group be sure to set it to use the appropriate policy sets and storage sets, most likely the same ones as the group where the user is currently assigned. That way the only change is the quota assigned.
Check this article for more about using groups to manage users:
https://syncplicity.zendesk.com/hc/en-us/sections/200516604-Managing-groups