I like the little bit extra width of the...
I like the little bit extra width of the large, but plan on cutting length. Any special treating of the cut edge necessary?
Mike, the short answer is no, I would just use a knife and be done with it.
The reason is Polyethelene, as used here is a foamed thermoplastic, with a melting point around 220-260˚F, so if you try and take a lighter to the edge you will just make it hard and crunchy. You could try to use a lower temp heat device, like a curling iron or a heat gun, but I don't see much point in trying. I don't think the pad will absorb any extra moisture through the cut edge.
I just use a scissor.