Possible to force printer setup (paper size) in javascript?

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%;
}

You can do this in CSS using the @media print directive, no js required. You'll have to calculate what sizes relate to a 4x8 index card and do all the positioning yourself, but it will work. Also, since this is CSS2 it won't work in IE6. (see Joel's comments)

@media print {
  body {
    width: /*width of index card*/
    height: /*height of index card*/
  }
  /* etc */
}