Use code like this in your grid question:
<script>
$(document).ready(function(){
switch ([% ListLength(list1) %]) {
case 1:
$('#[% QuestionName() %]_div .inner_table').css('width', '40%');
break;
case 2:
$('#[% QuestionName() %]_div .inner_table').css('width', '60%');
break;
case 3:
$('#[% QuestionName() %]_div .inner_table').css('width', '80%');
break;
}
})
</script>
"list1" must be replaced with the name of the list of columns. The switch statement can be updated with different widths depending on the number of column items.