File size will not change. Render time will increase. And, yes, there are times when this is beneficial to change. The complete list of differences between the settings have been stated several times. Here are the Differences between Draft, Preview, Good, and Best
I render everything at best, from what I understand if you use photos with keyframes movement, and slo mo, it is a benefit,, I figure what the hell I like the sound of best over good so it is always set at best
Ok, I tried a test with a 5 minute video clip. The render was done using Mpeg2 for DVDA. I set the qualitry to "Best". Render took 15 minutes.
I then rendered the video with the default, (good), and the render took 20 minutes.
Now, why would "Best" take less time? Also, I burned a DVD with the two different files and the "Best" rendered file would not play through without skipping and stuttering.
Why would this happen? Could it just be an unsual problem? For some reason I vaguely remember doing this before with similar results.