An older dog having accidents is usually a change, not a habit. That makes it different from training, and it changes what you need from a pad.
Start with the vet, not the pad
Incontinence that appears suddenly in a dog who was reliable is worth a conversation with your vet before it's managed at home. A pad handles the floor. It doesn't answer why something changed, and we aren't going to pretend otherwise.
What actually matters for an older dog
Reach and grip. A dog with stiff hips won't cross a room at three in the morning, so the pad goes where they already sleep. The anti-slip silicone dots are doing more work here than anywhere else, because a pad that slides under a dog who's already unsteady is worse than no pad.
Which size
32 x 36 in (81 x 91 cm) suits most large-breed seniors, and 36 x 41 in (91 x 104 cm) gives a big dog room to lie down fully rather than aim at a target. Older dogs don't place their feet as carefully, so the extra margin is the point rather than a luxury.
You will wash it more often
Overnight use means a fuller pad, and independent care guides for the category recommend washing every two to three uses for odour and bacteria rather than waiting for capacity. That's the case the 2-pack was built for.
The washing detail is in the care guide, and all eight sizes are on the washable pee pads page.