pantheon-files folders do not show its size, like files do.
However in properties of the folder size is shown:
Is there a way to display folder size directly in file explorer?
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The devs should correct me, but I don't think this is possible. I think you will have to file a bug report on launchpad.
The reason will probably be that it requires a lot of disk reading to find the disk usage of everey directory. For an extreme example opening
/: It will have to recursively read the entire filesystem.