I like a 10x10 tarp for solo use. It's so big, I can walk in/out at a crouch, dress in a kneeling position, and leave the edges 10-12" up off the ground even when the rain is coming down at 45 degrees. I live where it's hot and humid, so breezy is good. If you live where it's chillier or have blowing sand, the big size would allow you to pin the wind-ward edge right to the ground, and still have good coverage on the other side.
The big size also allows me to make a 3 sided pyramid when the rain storm has changing wind directions, or the temperature drops 20 degrees colder than expected. For using the tarp as a pyramid, having tie-outs 2-2.5 feet from the edge is useful since you can use it to get a lot more headroom closer to the rear wall. This makes it "almost, but not quite" big enough for 2 people to use in thunderstorms with little worry about the person near the door getting wet.
The 10x10 fits 2 people well in calm rain, but makes guessing how hard it might blow much more critical. ie: Will pitching the edge 10-12" high be OK, or does it need to be 4" high ? If you are willing to crawl in/out of the tarp, and don't need more headroom over the sleeping bags than someone laying on their side propped up on their elbow, then pitching the edge 4" up will be fine and still not require a bivy to say dry (unless you guess wrong on the wind direction, and it comes in the end of the tarp).
Your bivy will still useful with a bigger tarp to stay warm in colder climates, or the shoulder season of warmer ones. It's about 10 degrees colder feeling in a tarp due to the wind chill.