House-training runs on repetition, and a pad that moves around breaks the repetition. Most of what makes pads work for a puppy is placement, not the pad.
Put it in one place and leave it there
A puppy learns the spot, not the object. Moving the pad to keep it out of the way restarts the lesson every time. Pick somewhere you can live with for a few months, near enough that a puppy who suddenly needs to go can reach it, and away from where they eat and sleep.
Why washable changes the maths here
Puppies go often, and that's the stage where a disposable habit adds up fastest. Outside research puts disposable pads at $180 to $255 a year for a dog going through two a day. A washable pad is bought once and washed, which is the whole argument, and it ships as a 2-pack so one is always down while the other is in the wash.
Which size to start with
28 x 32 in (71 x 81 cm) is the usual starting point for a training spot. Go up a size if your puppy is a large breed who'll outgrow it within months. A dog needs room to turn and pick a spot. Too little room, and the pad gets missed at the edges rather than soaked in the middle.
The honest caveat
The edges are bound, with no loose corner to grab. Puppies still chew everything, so watch a new pad the way you would any fabric in the house. If yours is a determined chewer, that's a real reason to think twice.
Keeping it working
Skip the fabric softener, wash warm below 40C and tumble low. The full reasoning is in the care guide, and the sizes are set out on the washable pee pads page.