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).

Very noticeable.  In the printing world, 1 mm is huge.

My Incursion aliens are the same way (in both of the copies I bought), and for me this causes an additional problem:  About a month ago I bought special card sleeves designed specifically for FFG CE alien sheets.  They fit the base-game aliens perfectly, but the Incursion aliens are too big.  This is especially disappointing since, planning ahead, I spent over $30 on sleeves (for both the cards and the alien sheets) to make sure I would have enough even if there were another two or three expansion sets after Incursion.  Very disappointing.

Some print shops have machines that can bulk-cut large stacks of paper to accurate measurements. Although I don't know if they can chop off only 1mm. Last time I checked Kinkos would do it for about $2 per cut (if I recall correctly).

You can also fan the alien sheets to conceal the width difference. 

Just_a_Bill said:

That kinda annoys me because I am sure I have the same sleeves. Well I will probably take it to a print shop and see if I can get a mm cut off of either side. While I am thinking about it are the Arkham Horror Character and Elder One sheets the same size as the Cosmic Aliens? Also wondering the same about Descent as well.

Kobold Curry Chef said:

Nah, that's too easy.

Why not just deal out the flares instead and pick the alien power according to that? If you have 4 to 5 players, deal out one flare each and that is the power (each flare has the color-coding system on it as well) add an extra 6 to 5 flares to make 10 and add to the Encounter deck.

Yeah isn't the easiest way to just shuffle the flare cards and deal it out.  I mean the rules actually tell you do that.  Not that you can't decide to do it differently, but it's written right there...

I do deal out flares at the beginning of the game. It's selecting a new alien during the game where the issue occurs.

Macronx said:

We do the same thing during the game as at the beginning .... when needing a new alien we shuffle the un-used flares and make the choice out of that.  If it has Game Setup text or such, we simply draw again.

The thing is, a bunch of players don't like doing that because any flares filling out the deck's 10-flare requirement make those aliens unchoosable.

Toomai said:

... and, you're also giving up the chance of picking an alien whose flare is already in the deck.

I didn't notice a difference in size, but my cards do have a noticable difference in color. If look at the stack of original alien powers from the side, they look white, but if you look at the expansion alien powers from the side, they look black. This makes it fairly easy to look at the side of the stack and see what is coming up next.

I find the differences in size and colour only show up when the cards are squared.  When I'm shuffling them I can't notice at all, so I think in practice we won't have a problem.