I'm certainly no expert in making tarps, and wouldn't try even if I knew how to thread a sewing machine. But I do know that any question that includes "new to tarps" and, "girlfriend" must be answered with the perspective of 80% ultralight philosophy and 20 percent Dear Abby. Don't go small.
My wife -- who I still think of as my girlfriend -- and I use a 10x12 and love the space it provides, especially when it is raining and cold here in Southeast Alaska. I've been hinting at the purchase of an Oware 10x10 for our shorter and "drier" trips. She will come along, as she always has, once I can prove that we and the backpacks, or drybags if kayaking, will fit underneath.