you cannot size a background-color, but you can give a specific size to a gradient which is treated as a background-image.
here's what you could do: http://jsfiddle.net/JkhCs/
body {
font-family: "Helvetica Neue",Helvetica,Arial,sans-serif;
font-size: 14px;
line-height: 1.42857143;
color: #333;
background-color:transparent;
background-image: -webkit-linear-gradient(red, red);
background-image: linear-gradient(red, red);
background-size:80px 80px;
background-repeat:no-repeat;
}