back ground size

I am curious what your intention is. backgrounds are meant to (usually) encompass the entirety of the container they are placed in. So, for example, a background of a website will take up the entire page, whereas the background on an element will take up the entire element. Unless you actually are asking how to re-size an image, I'm going to proceed as if you were trying to set a background to a specific size for a specific element on the page.

the way I would do it is to make sure the container is 20px by 20px. In the CSS it would look like this;

container {
 height: 20px;
 width: 20px;
}

then I would make sure that the background is the correct size by using CSS to edit it like so;

 background-image {
 background: url (' ') no-repeat contain;
}

the above CSS will make the image scale down to the size of the container, without losing any quality.