Patrick,
When you put in the reference for the picture, you can set the width and height. They are part of the HTML <img tag. Here is an example.
<img height="200" src="put the address of the image here">
This will force the image to 200 pixels high and will automatically set the width to keep the original perspective. So you usually will only set one or the other. You should typically set the larger of the two, either width or height, to a set size and let the other follow.