Yes, the Specification Manager can be used to modify the data of any of the planting types, where changes apply to every instance of the selected specification. This page - Global Modifications – Single Species
- talks through using the Specification Manager.
However, if it's the size of the block that you wish to change, rather than the specification, the easiest way to do that is just to use the Properties panel. If you want to update all instances of a certain block, you can select one, right-click and press Select Similar.
That will select all copies of that block. Then open up the properties panel (available on the View ribbon, or on the right-click menu) and adjust the Scale X, Scale Y and Scale Z fields.
I hope that helps, anything else, please feel free to ask.