They don't need to be the same size. In fact, when tanks are drilled, the drain is usually much larger than the return.
You don't need to put a ball valve on the drain line, but you can if you want to. A ball valve is necessary on the return line.
When you plumb the return, it's also a good idea to make a T from the return pump. One end of the T goes to the main tank, the other end of the T goes back to the sump. You put the ball valve on the line that goes back to the sump. This way, you can control flow by diverting water back to the sump, without putting back-pressure on the pump.