With plants there are 2 ways:

You can either change the height of the plant in the plant editor (select the plant you placed in the scene, click on the wrench in the top right corner and it will take to the plant editor, where you can change "Tree Size" in the "General" section) or you can scale the plant (and most other objects) using the Motion Tab in the assembly room -- that the default room (hand symbol) where you add and pose your objects (select the object you wish to scale, and click on the Motion Tab in the properties panel where you see "General:Motion:Modifiers:Shading:Effects" tabs; Transforms section of the Motion tab has 3 sections: Position in scene space, Rotation around Hot Point and Scale -- you can alter the size in the scale section).

The same scaling can also be done using either the universal manipulator -- looks like a little Saturn in the left hand side vertical tool bar, or the Scaling tool -- looks like 2 arrow-heads pointing in opposite directions, same tool bar. Since this is essentially same as the Motion tab scaling, I am not listing it as a separate way to change the size.