I have used a 5×7, 9×7, and 8×10. I think that 10 feet is the ideal length for a tarp using no bivy. That extra foot or 2 makes a huge difference with wind blown rain and splatter. With a 8 foot long tarp, a typical 6 foot person has 1 feet of buffer at the feet and head, with a 10 foot long tarp you have 2 feet at the feet and head, so its like having twice the protection buffer. I find 8×10 the perfect size.
Tarps are not the best option for windy storms. They are best for camping in forests where the wind is cut down significantly. You can pitch a tarp nice and high with lots of living space while the rain pours down on you. It is possible to use a tarp successfully in a windy storm, but you need to pitch it very low to the ground – so low that you have to crawl to get under the tarp. That is a very uncomfortable and cramped shelter to deal with, but it will keep you safe. A pyramid shelter is a much more practical and effiecient design for windy rain.
A properly sized tarp is not much lighter than a one person pyramid shelter unless you are like me and don’t use trekking poles (i need to carry a separate pole for my pyramid shelter). The value of a tarp is in it’s adaptability and versatility, you can pitch it as a large canopy above you, a lean-to to block wind from one direction, or closer to the ground for bad weather.