The width does not have much to do with whether it will be heavy
enough. If you make it 2 feet wide, all the weight will be on a 2 foot
wide piece of lawn.
If you make it 4 feet wide the weight will be on a 4 foot wide piece of
lawn. Exactly the same amount of weight per foot.

If I were making one I would make it bigger in diameter. Probably
about 16 inches in diameter. I would make it longer than 2 feet, but
probably not 4 feet. I might have a problem pushing one that was 4
feet wide.

Dan