In the movie Angels and Demons, after the death of another cardinal (fire) Robert Langdon's clothes are all in blood. Camerlengo gives him a black shirt afterwards. When Robert is back with the shirt Camerlengo says "aren't you surprised that it suit you?" So I don't know if it is just a little joke but I thought it would be a clue about something. What does he mean by that?
You are right that there is a bit more to it, however, not a deep foreshadowing either, but just a little friendly joke (for lack of a better term).
First of all, the Camerlengo doesn't specifically talk about the shirt's size, what he actually says is
Would it surprise you to find those clothes suit you?
So he says the clothes generally suit him. And this is directly in relation to the fact that he actually provided Langdon with the clothes of a catholic priest. You can see them in the scene where Robert finds them and which explicitly shows the characteristic white collar worn by catholic priests.
And even more than that, you can see how Robert just grabs the collar and throws it aside and the way and look with which he does that. This again highlights the fact that he isn't actually much of a religious person at all and especially not a big fan of the catholic church as an institution. His supposed disregard for the catholic church (as well as vice versa) is adressed at many points in the film (e.g. the reluctance of Vatican to let him into the archives and the conflicts he gets in with Commandante Richter).
So when the Camerlengo says that those clothes "suit" Robert, he plays with this attitude of Robert towards the catholic church, now ironically being clad as one of its members. It might be part friendly jab, part attempt by the Camerlengo to lighten Robert's stance towards the church a bit, and part saying that he might not be as different from them as he thinks. But at the end of the day, it was a friendly little "joke" from the Camerlengo, which Robert eloquently responds to with another little wordplay.
It would surprise the hell out of me.