The OP mentioned cooking, probably not something you want to do under the tent fly (assuming the OP has a double wall tent). Food odors attract varmints (bears, skunks, raccoons, squirrels, chipmunks, mice, etc.). I'd be especially reluctant to use a tent that had been used around cooking when in bear country.

I'd go with the 6' x 8' Campmor tarp (haven't noticed any wear around the grommets) or a silnylon poncho. silnylon poncho. The latter (close to the same size) would be a little lighter and could also do double duty. Both are made by Equinox. The 6' x 8' tarp easily covers 2 adults, 2 children and the cooking area.

Assuming that everyone wears appropriate rain gear when it rains, a small tarp (maybe 3' x 4') would be more than sufficient to shelter just the cooking area. Mountain Laurel Designs makes a 4' x 4', 4 oz. silnylon "dog tarp" which would be ideal for this purpose. Or you could make your own. It can also, of course, be used as a dog shelter, although probably not while you're cooking!