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Get the document filepath: Document.PathName
From here use the default windows FileInfo.Length to get the size of the file in bites.
Be aware that the pathname can be a BIM360 path (e.g. BIM360://mysharedmodel.rvt, which you won't be able to easily map to a on-disk path). The pathname may also be "UNTITLED" or something like that for new (unsaved) files.
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