There are a bunch of posts on Tent Ground Cloths. They're all pretty easy to make, just as this one is.
Then why another DIY ground cloth making post or in this case Instrucructable one might ask?
My answer is simple, because there seems to be plenty of confusion about tent ground cloths, mainly the primary purpose of a tent ground cloth. With out knowing the primary purpose, you can't make one correctly, now can you?
Somewhere along the line, the primary purpose got lost. And I don't care what anyone tells you, what company makes what claim. The primary purpose of your ground cloth is simply to help protect your tent floor from sticks and stones and punctures. It is not to weatherproof your tent floor. It should already be weatherproofed. If not, that's anther issue you have all together. And should be remedied before your next outing.
That's not to say you can't add additional waterproofing to your ground cloth. Of course you can. But it must be limited to the floor dimensions. Preferably the waterproofing on the ground cloth should be your tent floor dimensions less an inch boundary all the way around the floor perimeter.
That said, your "ground cloth" can be as big as you want it to be. It should allow water to run through it, not soak up water, dry quickly and help protect your tent floor from the elements on the ground. Which you hopefully swept and cleared the area you plan to pitch your tent of all: Sticks, Twigs, Rocks, Logs, Stones and other things you might be uncomfortable lying on.
You can have a ground cloth 10 feet by 20 feet large with a tent only 3 foot by 7 foot if you like. But If the whole thing is water proof, guess what? First rain, your tent and everything else on it, will be sitting in water. And that is not what you want. Well, I don't, maybe you do...?
Hopefully you have a clear understanding of what your ground cloth's purpose is for. Lets make a simple yet effective one.