I have dozens of board games and typically do NOT place any of the cards in sleeves. I actually bought a full set of sleeves for Terraforming Mars, tried them, disliked them, removed them (sold them 2nd hand for half of what I paid). However … I now have an EXCELLENT reason to use sleeves for this game! There are three additional characters with nicely done card images that you can print on your own printer. However, mixing them in with the thicker cards that come with the game does not work well at all. However, it you put all the Character cards into sleeves (both the originals and the additionals), you can shuffle and mix them together just fine. Just be sure to print both the front and the back of the cards! Here is where you can find the additional Characters:
=> Additional Characters (printable cards)

But wait … there’s more!
There are SEVEN “phases” to each day (each round) in the game. I tend to forget where I am at in a round, so I made 7 gameplay “phase” cards that I flip through as I play through the game. You can print those 7 cards on one sheet of paper back to back, or on two sheets of paper. Either way, after you cut out the cards, place them in a sleeve and it will be much nice flipping through them as you play the game. Here they are:
=> 7 Cards for Daily Phases

You can find standard size card sleeves from a variety of manufacturers and in a variety of places. The size that I have that works well is 66×91 mm. I got non-glare, non-reflective so they are easier to read. However, this size sleeve is slightly too big to fit into the box insert.
If you’d like to sleeve all your cards but still want them to fit into the game box insert you need to use a sleeve called an “inner sleeve”. These tight fitting sleeves are great for all the cards in the game EXCEPT for the Character cards because they are so thin. An additional character card printed on normal paper works well inside a typical “normal” sleeve, but the tight fitting sleeves are too thin and the cards you print on your printer just don’t work well in them.
See this article about the tight fitting sleeves:
=> Sleeving the cards (I didn’t sleeve any other games, but I did sleeve this one)