Pic to Poster

I’m looking to print a couple of picture and go to Kinko’s or something to get them posterized (Not a verb I know, stfu).
Of course I don’t want the quality to drop too much so how could I tell how big I can print it out without worrying about degradation. I’m assuming it has to do with resolution but I know so little in the domain that I don’t want to make premature assumptions.
My apologies if it’s been asked before, if so just link me.

Edit: I just realized this should probably go in the stickied Image Manipulation thread. If so, just close the topic/delete it.