Oops! LOL
He said it was the biggest one he had. I'd guess either a #9 or a #12. Like everything else though, I imagine it depends on how think a bundle you want to cut each time. Since you pre-cut the jig, you could judge what blade to use by the thickness of the sandpaper bundle I guess.
Hey, if you decide to punt it, maybe someone who couldn't otherwise afford a mop will benefit. For me, it would be a lot easier (and less annoying) to go to Klingspor's web site and just buy the pre-cut set but I know some people get a lot of satisfaction making their own stuff. I guess it just depends which category you're in.