Resizing objects in Mission Editor is simple.
Go to Window>World Editor Inspector and click on the object. Then simply go to Transform>scale> and set it to your liking.
In the ("_" "_" "_") for the scale, the first number represents the x axis, the second the y axis, and the third the z axis. Therefore, by making it ("0.1" "0.1" "10") you are making an extremely thin and tall tree.
The same can be done with the rotation of the object, but for the position it is easiest to use the x, y, and z axises to drag it around instead of messing with numbers.
This was taken directly from my old tutorial.