Based on guidance from the IRS:
The cost of room and board qualifies only to the extent that it is not more than the greater of:
-The allowance for room and board, as determined by the eligible educational institution, that was included in the cost of attendance (for federal financial aid purposes) for a particular academic period and living arrangement of the student, or
-The actual amount charged if the student is residing in housing owned or operated by the eligible educational institution.
You want to ensure that, in the event of an audit, you are able to meet the aforementioned requirements concerning room and board with appropriate documentation.
Your educational institution should publish estimated room & board expenses. For example, Rensselaer Polytechnic Institute has estimated Room and Board of $14,630 for the forthcoming 2016-2017 academic year. If I were attending this coming year and my combined expenses fell within $14,630, and I documented said expenses, they would constitute qualified expenses. To be conservative you should contact your institution's Financial Aid office for a breakout of estimated room vs. board expenses.
For more information you can also see, "The Best Way to Save for College," page 52.