Add list / library storage size property
Currently there is no way to find out the exact library / folder or list size. If you want to know the storage size used, you need iterate through each file, version of it and recycle bin, which could take ages in a environment with +10TB. Also the StorMan.aspx is not accurate and flagged as deprecated by MS.
Matt Groves commented
As above, iterating through a site with several large libraries simply isn't feasible, either exposing the storage management api in SPO or adding a property to list/libraries for the size is needed.