This is what I do to resize capped collections:
db.runCommand({"convertToCapped": "log", size: 1000000000});
I already have a Capped Collection named "log". So I just run the "convertToCapped" on it again, specifying a new size. I've not tried it on reducing the size of the collection. That may be something that you'd need to use Scott Hernandez's version on. But this works for increasing the size of your capped collections without losing any data or your indexes.
EDIT: @JMichal is correct. Data is preserved, but indexes are not and will need to be recreated.
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