I asked Rightgames the same question about the Continents. They answered they will check it out. I'll let you know. But I'm nearly sure that the square box contains 88 x 63.5 mm cards and the rectangular one has another size (don't know the details).
I have the square box for the base game, and used 63mm x 88mm card sleeves and they fitted perfectly. I don't know about the rectangular box either.
I'm intending to get Continents, so I'll be interested to hear how to tell which size cards are in the box!