In the transform palette just to the right of where you enter the height and width, there is a little chain link, click on that and it should size the height to what ever width you enter and vicea versa. either that or you can hold down shift when you grab the corner of the bounding box.
your right in Illustrator 10 there is no chain link. However in the the transform pallette after you type in the size of either the width or the height hold ctrl and hit enter and it should size it proportionally.
if you don't have your rulers up yet, ctrl-R will display them. You can also click & drag from the corner where the height & length rules meet & this will allow you to reset the ZERO of each ruler to a defined point, like the corner of a layout or object.
Then you would also benefit from the info dialog box. Under the "window' pull down menu, choose 'show info" this box will tell you height & width of any object you select... except type measures wrong untill it is converted to outlines. I just draw a box around type sometimes to measure it.