My measurement of the size of the Khan of Khan cards is: width 63mm x length 90mm.
To protect the Khan of Khan game cards I used "FANTASY FLIGHT Standard Card Size Board Game Sleeves Clear 63.5 x 88mm 50ct FFG" (this is the title advertised on ebay). These held the Khan of Khan cards quite reasonably, and two packs of 50 or one pack of 100 will house the game cards.
The size on the packet (63.5 x 88 mm) is the size of the cards the sleeves are supposed to hold. The size of the sleeves is 66 x 94 mm and holds the Khan of Khan cards quite reasonably. I've been advised Fantasy Flight Game sleeves are usually larger (by 1-2mm) than the card they enclose. Anyways, these cards fitted for me.
Note the above type of sleeve is different to those labelled American Card and European Card sleeves.
I also tried Mayday sleeves which I've used successfully for other games, and turn out very snug, but they were just a bit too small for these, and I stopped trying at the first sleeves which worked.
B.