The problem is that it is impossible to ensure by rules that all posts will be high quality, since high quality is a subjective thing.
Luckily, the problem is already solved, since the voting phase is after phase 1, so each voter can decide whether he finds someone's posts high quality or not. If someone makes 6 bloated posts and someone else makes 6 well-thought posts, people will generally favor the second one (unless they are too lazy to do some research before voting).
Anyway, if someone wants to write high quality posts with 50 words, rather than 200, there is a simple solution. Write a 50 word high quality post. Add 150 words of examples or further explanation of your ideas. Separate the 50 word part from the 150 word part, for example like this:
50 words yadda yadda yadda
150 words yadda yadda yadda
yadda yadda yadda
yadda yadda yadda
This way, if someone doesn't feel like reading bloated posts, he can simply skip the longer part after reading the 50 word summary, since he already understands what you are writing about.