What I did was store a bunch of the same species and recorded their sizes. I used birdwing butterflies, but any rare bug should do well. You can't do anything about the quality rating, it's completely random. I even did a reset to test this. The same bug will get different results for its...
I always thought that there was a certain amount of trees that were allowed per acre. (Like 16 or something) Any more trees than that and the sapling will die. I thought that was the case as after I counted trees per acre I did not have any trouble with dead saplings.