Most patches are round, but glued kits often contain oval patches too. These tend to be of limited use - almost all punctures are point piercings of your tube, so a round patch will work just fine.
You shouldn't need a large patch either, provided you get a good seal, although glueless patches tend to be less airtight than glued ones, so some extra material around the point of piercing may be helpful. A larger patch will also help if you don't aim well and the actual hole is off-centre from the patch.
One case where a larger, oval patch might be better is a "snakebite" puncture, caused by the tyre bottoming out on the wheel rim. You'll see two parallel cuts a few millimetres apart. Here, a slightly larger patch should work better.