There are two different types of quotas. The first is the server-side quota, which is the maximum allotted disk space (or maximum storage space assigned) for each user. The settings can be found under RBS Manager: Accounts section - edit User details.

By default, it is unlimited for each user created.
It is possible to modify a registry key setting to assign a DefaultQuota value. When a new user is created, the new quota is read from the key and assigned automatically.

To edit the DefaultQuota value, bring up the Registry editor and browse to the following branch:

HKEY_LOCAL_MACHINE\SOFTWARE\Quantum Tech, Inc.\RBSServer\RBSMgr -
"DefaultQuota" - "Unlimited" (by default)

Here is where you change the default quota; it is measured in megabytes.
So, if you type in 500, the default quota for each new user will be 500 MB.
Second is the SessionQuota, which is the client-side quota (also set to Unlimited by default). It defines the maximum amount of data a client can transfer in a single session. If, during a backup session, the amount of data being transferred exceeds the SessionQuota limit, the client will stop the transfer and finish the session.