This is a very interesting exercise, Fidur!I can see a monster standard deviation, wich allows me to say that when you submerge in a pic of a tree, with no scale refferences, you can imagine the scale that better fit your expectations, and it seems those can be different for different people....
I want to think, that when you introduce a scale refference in one tree (an animal for instance), you're setting a scale..
18 inches would be my guess also.18 inches
yup18 inches would be my guess also.