I had some of FF's 'standard card game size' sleeves hanging around, and those were a bit too big to fit on the cards in this game comfortably. What should I use that would fit more snugly? Would Ultra-pros be better? Or another of FF's sizes?

Are you using FFG Gray/Clear sleeves? You can see the recommendation behind the box.

So I got those. And they were a bit too big. I guess I'll have to deal with floppy sleeves.

Too big? Are you sure you got the ffg ones? Mine fit as they should