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Using the below, you can determine the size of individual files.
svn list --verbose --recursive http://svn/repo/path
Maybe you can look around this command to find a way?
I've tested the PowerShell solution and it just needs a small correction:
([xml](svn list --xml --recursive https://svn/repo/path)).lists.list.entry | measure-object -sum size
It some time to get the xml, but works very well.