Maybe I misunderstood, but I took this to mean you have a select question that is using a constructed list and you have the check box settings set with the number of columns at 2. However, if your constructed list being used has 6 or less options you wanted the number of columns to only be 1, not 2? Is that right? I'm not sure if there is a way to change that setting based on the list length, but if that is what you wanted to accomplish you could create 2 questions (one set with 2 columns and the other with only 1 column) and skip the questions based on the list length of your constructed list. Now, this would require you to combine the responses on the back end, but it would accomplish what you wanted for the functionality of the survey.