Hi. I found that the best Mayday size to use are the Orange package, which I believe are called USA Chimera. The blue standard Euro sleeves are too big.
For my copy (Eagle Games ed.), I found the Mayday "Standard USA Game Size" (purple, 56x87mm) premium fit quite snugly, which makes sense with malzspiele's statement that the cards are 56.5x87mm.
I found the orange "USA Chimera" premium sleeves (57.5x89mm) a bit big, but tolerable.
Left: Mayday USA Chimera Premium
Right: Mayday Standard USA Game Size Premium