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