I don't thing there's an extension for it yet.
However, you could easily use a list object to load the contents of a folder, run a fastloop to check the size of each individual file in the list, and then just add them all up.
It's a bit more hassle if you have folders within folders though.
folders don't have size info attached to them anyways, so an extension would have to do it the same way. Scan the directory and record the size of all the files.
You could probably get it at a similar speed as an extension. Just with less code
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