
With my medium size double coated breed where I tend to have around 4 - 7 puppies a pad a bit larger than a sheet of A4 is plenty.
This means I can at first put it in the middle for the pups and the bitch can lie on any side without having to lie on it, and get hot.
The pups in my breed tend not to want extra heat by 10 - 14 days, it is plenty for them to pile on before then.
At two weeks I keep it on only at night in one corner for the pups benefit as by then Mum will choose to lie outside the box most of the time.