Set your widths and heights to percentages, not pixels:
.tall
{
background-color:lightgreen;
border:dotted 2px green;
opacity:0.5;
filter:alpha(opacity=50);
height:100%;
width:10px;
position:absolute;
left:0px;
top:0px;
}
.wide
{
background-color:lightblue;
border:dotted 2px blue;
opacity:0.5;
filter:alpha(opacity=50);
height:10px;
width:100%;
position:absolute;
left:0px;
top:0px;
}