The alien sheets in my copy of the base game are 114mm x 171mm. The sheets from Cosmic Incursion are 115mm x 172mm. This is slightly annoying since it makes them very conspicuous in the deck of aliens (if you need to draw a new one). Yes, 1mm is noticeable.
However, they are still playable. I just modified the way to select a new alien. I used to have the players cut the (alphabetized) alien deck several times and draw the top one. Now, I have one player cut the deck and the player who needs a new alien call out a number between 1 and 10 (or so) and take that alien from the top. It still plays relatively fast and avoids any "temptation" to cut to a new alien. Although you could call out a number between 1 and 70 (minus the number of players in the game), it's much quicker to count to 10 than to 42 or 69 (two all-time favorite numbers).