Look at this CSS3 examples from http://www.w3.org/TR/css3-page/#size:
/* style sheet for "A4" printing */
@media print and (width: 21cm) and (height: 29.7cm) {
@page {
margin: 3cm;
}
}
/* style sheet for "letter" printing */
@media print and (width: 8.5in) and (height: 11in) {
@page {
margin: 1in;
}
}
/* A4 Landscape*/
@page {
size: A4 landscape;
margin: 10%;
}