Hi Lisa,
Depending upon what image you are trying to adjust - whether it be the main product image or thumbnail image you will have to get the css class for these images. To do so simply go to your file manager and look at the css file. You will see classes that have the name 'img' in them. That is the class for the image in that section.
To change the height and width you then simply need to add height and width attributes to that css class.
For information on how to do so please checkout -
http://w3schools.com/css/css_dimension.aspHope this can help
Joe