Use the boundingbox plugin, and add an X on the bottom of the box. Group everything and then reposition the center point of the X on the origin (it's a clickable point).

If you explode the instance in your model, the component will still remain in the file. Go to your component browser (Window menu), navigate to "In model" and purge it from under the popup menu triggered by that little, blue icon next to the search field.

OR keep it for later.

To center an object exactly on the x,y,z - there are 2 (at least) ruby scripts that do this, and they each do it slightly differently.

Move To Origin
This script will move a component or group to the origin. It places the component axis on the model origin. This was written by Todd Burch and is hosted on smustard.com.

Center on Origin
This move a group/component to the origin, but it placed the very centerpoint of the group/component onto the origin. I wrote this for someone making objects for games that needed to have their centerpoint's located on the origin.

So they are slightly different, but both work well. For scaling, use the tape measure tool. It works like this:

Measure something that you know how big it should be. As soon as you measure it, type in the desired length and hit enter. It will rescale the entire model! If you don't want the entire model to be rescaled, then turn your object into a group/component first. Double click on it to "edit" it. Then do the measuring tool scaling trick. It will scale all objects within that component only - not the entire model.

Hope that helps,

Chris

Depending on what you really want to do, I believe some workarounds do exist in SU. Just too tedious.