It seems that people who regularly behave as described by the original poster tend to get warnings from moderators anyway. To me, at least, it seems that the problem is not the lengthy quotes, which can be legitimate and useful; rather it's the poster quoting expansively and then putting "I agree" at the end. These kinds of posts are against guidelines anyway, so a new rule is not really needed.
Also, it would be a pain for the mods to enforce, and a character limit would be really frustrating (if it cut you off mid-word, for example).
I do agree that it is really annoying when people do that, (especially when the post being quoted is just above!) but I personally feel that the mods already police this issue enough - we also wouldn't want the forum to become like a police state!