When you want to get some pretty good data on using social media for marketing, one of the best people to turn to is Dan Zarrella, Hubspot's social media scientist. "Mad scientist" may be a better phrase for him, but he's crazy like a fox when it comes to Twitter.
This latest round of insights comes in the form of how to get retweets. Size is important as can be seen in the graph above. What's the right size? 100-115 characters appears to be the sweet spot. This can be attributed to a few things. First, longer Tweets can be retweeted the standard way, but when they're manually retweeted (such as "RT @0boy...) then the longer Tweets can't work as well. More importantly, people know that they will not be as easily able to be retweeted themselves if the Tweet is too long.
Another reason for this is quality of content. With the limited space in Twitter, it's hard to say things that are profound, funny, or generally retweet-worthy until you get into the longer format.
Lastly, tweets of this range seem to look better. They may or may not include a link. Whether they do or do not, they appear very nicely in Twitter apps and in the stream in a way that is psychologically appealing. It may sound simplistic, but shorter tweets seem too short and longer tweets turn people off for the reason given above.
At any rate, this data is compiled from a data set of 1.4 million randomly selected Tweets. It's as comprehensive as they come. One thing that should be noted: the length of the Tweet is infinitely less important than the engagement of the account itself. If nobody's listening to you, no measure of science is going to get you more retweets.