Ok, so I found out the cards are bridge sized. The Mayday 2.25 x 3.5" sleeves fit perfect.
The FF ones probably fit a little bigger as even though they're marked with the same imperial measurements, the metric (mm) measurements are marked as being slightly larger than the equivalent Mayday sleeves so they'll be a floppier fit.
I can't remember from the kickstarter campaign if the cards will still fit in the box when sleeved with premium sleeves?
They do. I used FFG sleeves and fit all cards (base + expansions) back in the box, but not with the original insert. Toss the insert. I cut off part of the insert to put between two stacks of cards to keep them from mixing while in the box.