Where you have (in your original code snippet)
<div><input type='checkbox' id='s90' name='s90' value='s90'
Change this to
<div class='s90InputField'><input type='checkbox' id='s90' name='s90' value='s90'
and then add a new rule AFTER the INPUT rule - in fact add this at the end
.s90InputField {
width: 20px;
}
Obviously you can adjust the width to suit.