With Canopies and Tarps, you can find the perfect tarp at the exact size you need.
A word of advice if you want to try and attach two tarps to reach a certain size: Don't.
A tarp is effective at protection against the elements because it features a large, uninterrupted stretch of high-quality material. Attempts to join two tarps require a bunch of stitching or heat treating, which is hard to accomplish successfully without the right tools.