You can just use the standard HTML/CSS methods for setting the size of the image.
<img src="image.gif" width="300px"> = 300 pixels wide
<img src="image.gif" width="50%"> = 50% of the width of the image's container
<img src="image.gif" width="50vw"> = 50% of the viewport width
If that isn't what you were looking for, please explain in more detail.