Sorry for the late answer. There is no easy way because subversion
only sends the names and type (file/folder) in the XML. I think it
would be possible to implement using a javascript added to the page,
running XMLHttpRequest (Ajax) with a HEAD request to each file and
then reading the returned Content-Length header.
It is also possible, of course, to do something like the log viewer
that runs "svn list --xml" and formats the result using xslt, but that
would not have the benefit och being displayed under the correct
subversion URL.
/Staffan