Is there a feed file size limit?
If you currently publish an existing FeedBurner feed, and the original source feed exceeds the 512K limit, FeedBurner will not update the 'burned' version of your feed until the original drops below the limit. For most feeds this will never be an issue -- but if you know that your content postings are regularly very large in size, you might want to keep any eye on the original feed's file size.
Note #1: FeedBurner should proactively notify you of a 'feed health' issue like this and help you avoid any service interruption. It's (still) on the list! If you use Blogger, you may follow these steps to reduce your feed file size below 512K if it currently too big.
Note #2: Publishers of podcast feeds that point to an MP3, torrent, or other binary file do not have anything new to worry about -- the size of the binary file that's merely linked from your feed has no impact on your feed's calculated size.