I recall speaking to Ventisit 2 or 3 years ago when I was considering buying a pad for my Performer Amigo. The guy I spoke to was very helpful, and suggested a size which was intended to be about 30mm smaller all round than the pan size - because he said that they spread out when in use. I didn't buy one at the time because I gave the pad I had a bit more time, and it's proved to be fine for my winter riding on that bike.
The Ventisit (style?) pad on my SAKI looks to be about 30mm too small all round, it looks to be too small, but in use it definitely spreads out a bit so you certainly don't feel that the pad is too small. You don't want them hanging over the edge in use.
If that helps......