Buy big! Its important that you buy big enough to protect occupants from wind blown rain and to prevent contact with the tarp. Mountain Laurel Designs grace duo would fit the bill. Available in Silny, spinnaker or spectralite. Great design (designed for backpacking not tank battles), well thought out and well made. Caternary ridgeline and perimeters, grommets to accomodate pole tips, 12 pegging points with easy tensioning linelocs. I have the spinntex version and am very pleased with it.