Theming long lists of checkboxes- How can I divide lists of checkboxes into multiple columns?
Quick and dirty:

.form-radios,
.form-checkboxes {
  margin-left: 5px;
}

.form-radios .form-item,
.form-checkboxes .form-item {
  font-size: 64%;
  width: 19%;         
  overflow: hidden;
  text-overflow: ellipsis;
  white-space: nowrap;
  display: inline-block;
  margin: 0px;
}


and play around with the font-size and width to your specifics.  If you want to restrict this to a specific form, just add its id to the mix, eg:

#FORM .form-radios, ...