The Details sub-tab is located within the Home tab of the ONE desktop application.
This sub-tab lists some useful information, such as the total amount of data set to upload to your account before being de-duplicated or compressed. It also lists the total number of folders stored as well as the total number of historical versions within the account.
The biggest thing listed within this sub-tab is the top 50 space consuming journals. This makes it a good resource to see, at-a-glance, if a file is taking up much more space than it should be.