length field in podcast feed

Hi,
We imported a bunch of sermons from Sermon Browser and now are using sermon manager. I noticed the length field in the podcast feed is 0 for all of the imported sermons. I have not found a way to update this other than redownloading a sermon into media manager and then using that. If I use the url to an mp3, it does not update or compute the size. I see that this is a separate field from mp3 duration as well. How would I update the size on these files?
Thank you!

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