I think it’s just that there’s no hard and fast answers. There are a lot of factors that determine how much or how completely something will felt. A lot of it depends on what wool you use.
You might be best to take a look at a bunch of patterns for felted items, take note of the gauge by which you can figure out the pre-felting dimensions (if the pattern doesn’t already tell you), also take note of the finished size and you should get a pretty general idea.