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Currently, the best way to do this is to make use of an add-on installed alongside FeedWordPress. In particular, I have a free add-on module called FWP+: Limit size of posts, which I think should allow you to do more or less exactly what you are looking to do. Take a look here: https://github.com/radgeek/fwp-limit-size-of-posts and let me know if it helps out with what you need. If you have any questions about getting it installed and configured, just drop me a line.