Helen,
Try this experiment:
1. Type helen into the classes field in a course you have in the authoring tool.
2. Preview the course.
3. Inspect the component using something like Chrome Developer Tools (Chrome menu > More tools > Developer Tools)
4. Locate your name. It will appear as a class in the div that contains the component.
I think what you want to do is to introduce/inject the class name via the authoring tool classes field, then add some styles to one of the less files. (I don't think it will really matter which, but you might consider using theme.less)
.helen .mcq-item .component-item {
font-size: smaller;
}
Use your class (.helen) with the appropriate class/es from the element you want to style. (This is all just example code; nothing precise.)