To see your total storage used, go to My Info -> My Plan & Storage.
To see the total storage for one folder, select “Folder Info” in the folder’s (☰) menu on desktop, or its 3-dot ( ⋮ ) menu on mobile. Press the “calculate” button to see the folder’s storage use.
To see how much storage an individual item is taking up, just go to the item’s detail view in the bottom right corner. On mobile select the 3-dot (…) menu in the next view and choose “item info”.