How to set a:link height/width with css?

add display: block;

a-tag is an inline element so your height and width are ignored.

#header div#snav div a{
    display:block;
    width:150px;
    height:77px;
}

Anchors will need to be a different display type than their default to take a height. display:inline-block; or display:block;.

Also check on line-height which might be interesting with this.

Your problem is probably that a elements are display: inline by nature. You can't set the width and height of inline elements.

You would have to set display: block on the a, but that will bring other problems because the links start behaving like block elements. The most common cure to that is giving them float: left so they line up side by side anyway.