Styling elements based on sibling count | Lea Verou
http://lea.verou.me/2011/01/styling-children-based-on-their-number-with-css3/
[css]
/* one item */
li:first-child:nth-last-child(1) {
width: 100%;
}
/* two items */
li:first-child:nth-last-child(2),
li:first-child:nth-last-child(2) ~ li {
width: 50%;
}
/* three items */
li:first-child:nth-last-child(3),
li:first-child:nth-last-child(3) ~ li {
width: 33.3333%;
}
/* four items */
li:first-child:nth-last-child(4),
li:first-child:nth-last-child(4) ~ li {
width: 25%;
}
[/css]