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.
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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.
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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.