Tracy, if you are using a modern web browser (Chrome, Firefox, IE 10+) try this CSS styling on your Checkbox Set Selector widget:
This basic concept worked for me in Chrome and IE11, but it may require some additional styling effort to get exactly what you want. Here's an extremely simple example:
Is currently not possible but the widget can be expanded with a rows and columns property.
I have some additional formatting issues with the checkbox set selector I'm hoping someone can help with.
Extraneous columns - with each selection made I am seeing an additional column added after the 1st column, which adjusts the entire layout. This happens with each subsequent selection as well and happens whether I have only a single column display or use multiple.
Alignment - With multiple columns, sometimes the checkbox and the checkbox set selector are appearing in separate rows
Border line - borders lines are visible around each checkbox as well as the associated text. If the above issues are addressed this may not be as much of an issue, but I feel that it amplifies the layout issues above.