I do not think it's worth trying to get everyone to agree on a universal length limitation.
A better, more flexible approach might be:
- Set some ridiculously high upper limit on how long a microblog can be before it becomes just a blog — say, 1024 characters.
- Within that limit, allow users to specify their own length limitation. This character limit they choose will be specified in their RSS feed as a whole, as well as per-item. Exceeding your limitation within any one item might cause the feed to fail some kind validation.
As readers, everyone can then (for example) set an upper limit on the feeds they would see. If I was only interested in Twitter-style tweeters I might not see "retweets" of those who write in more than 140 characters.