What size for a family of 6?

I'd love to save money and get the 15 inch, but then a friend with 2 kids told me if she had more than two kids she'd only get the 27 inch. There are 4 kids and 2 adults, and all 4 kids are in sports and other extra curriculars. We also homeschool so each will have lists for their assignments that day. I'm wondering if the 15 inch will be too small?