Converting the FileSystemObject's TotalSize Property
Definition: Returns the total size of a drive or network share.
Retrieve an object reference to the .NET Framework object DriveInfo for the drive you want to work with. You can then use the DriveInfo TotalSize property to retrieve the total size of the drive.
$d = New-Object -typename System.IO.DriveInfo -argumentlist "C:" $d.TotalSize
This does not work with network shares (i.e., \\atl-fs-01\scripts), it works only with mapped drives letters.