You really need to sense the scale of the scene. I've done scenes with inf terrains and dynamic ecosystems, and the trees looked small, but they were at the right scale for such vast scene. My advice would be to use some kind of ruler, like a cube fopr example, and a tree.
For example a vue cube is 6mx6mx6m, add a tree and check relative scales, and see if this tree's height is normal. The plum tree, for example is sometimes way too small in world units, and has huge embedded size variations. Once you have the right trees, you can populate. The default scale for ecosystems is 0.5, which is not right for large scale scenes. Up it to 1.000 ad you should have a believable population.