If I want to increase the size/height of every instance of a certain tree, how would I do that? I'm basically trying to make the forested areas of the game look like a rainforest.

Each tree is a reference to a TREE form with unique coordinates, rotation, and scale. To do what you're after with the CK, since tree forms cannot be scripted, you would literally need to edit every reference to the tree form manually.

A better way would be to tinker with the model itself, probably, so long as the scaling of the model would not offset the placed references.

Right. And what software would allow me to tinker with the model?

Wouldn't it be easier to duplicate an existing tree(because Im not sure if changing a non duplicated item, will change ALL that is being used in the game, which is considered a dirty edit), and then place it in the game world, click on it and change the scale?

How about adding a script to the tree reference? maybe you could have it scale them up at their instance creation?

It would be interesting if you could scale things with a script...imagine doing a http://www.youtube.com/watch?v=2e3vQmhm3Bc mod, where a wizard zaps you with a shrinking spell and reduces you to the height of a skeever!

Would give new meaning to "Master level difficulty"

As far as I know, size is whatever size it is as a model (Blender, 3DSMax etc).

However, you can scale things in the render window ... And doing that for every tree in game is like 1 minutes work, right?! ... Once the object is selected in the window, just hold down S, hold down the left-mouse-button and drag (carefully and slowly!).

You can do this with (almost) any object ... but becareful, because it almost always looks naff when you've done it.

So - Best way is to make new models ... Other than that, use the scale function of the CK

Scripting
I'm not falling over any scale/size functions ... but I've not looked that hard ... But, given the other stuff I have looked at, I doubt it is possible

TREE forms cannot be scripted, so a scaled down model would be the way to go as it wouldn't require cell edits unless there were instances where a larger model made the tree clip with rocks or whatever. That's bound to happen, even if the tree is only to be slightly wider in its diameter.