Setting storage limits for site collections was considered part of good governance when you were looking after you on-premise farm. In the cloud, however, you can have this done for you. Even though this not may sound like a good idea, it may well be one! Having storage limits managed automatically means that a site collection can initially grow in size to 25 TB (terabyte). However, unused storage space is automatically returned to the tenant storage pool. And this would not happen, if you have manually defined storage quotas for you site collections. So there is a fair chance that your run out of storage space more quickly when you manually configure site collection storage quotas than when you choose automatic storage management for SharePoint site collections. Additionally, you could monitor storage usage with a simple PowerShell script that at regular intervals verifies check’s each site collection’s size and reports any irregularities.