Use the Real Size
Save your graphic at the size you are using it within your Web page.
Alway make the image in your Web graphic file be the size that you plan to use it at within the page. If you'll be using it at several different sizes, make several different files each at a different image size.
You can resize an image using the image tag's width and height attributes, but in most cases all that will do is add overhead to the time it takes the page to display: