I saw the great ideas for flank steak below. That's helpful, but how much per person?
We are doing a lunch for 25 serving flank steak (at room temp or chilled?), roasted or steamed asparagus, some sort of potato... How much steak should we plan on preparing?
Don't purchase flank steak by the piece, as every piece will have a different weight. Instead, buy it by weight. One piece can weigh anywhere from 12-14 oz to 20-22 oz. The larger one might serve four, but the smaller one will only feed two, with shrinkage during cooking.
6 to 8 oz purchased weight should be good for most adults, so ten to twelve pounds will work for you.
Serve flank steak at room temperature. Slice it thinly, against the grain of the meat and on the bias (that is, perpendicular to the grain, and at a roughly 45 degree angle) -- given the relative toughness of the meat, this is essential towards making it more pleasant to eat..
See, the problem with your question is that if you make flank steak right, it's sort of irresistable and hard to stop eating, because it tears off in wonderful little stringy pieces that you can keep popping in your mouth . . . my mother made a fantastic version when I was growing up (she called it "London broil" but I have never found another London broil made the same way she did), nicely marninated, and crispy on the outside but with a range of doneness from rare to well inside so every bite seemed a little different, and, jeez, I could probably eat 2 pounds by myself.
Most flank steaks are about 1 - 1.5 pounds. For lunch I would assume 8 oz precooked weight per person unless you have hearty eaters. A lot depends on how much extra apps and such you are serving. However, since flank steak is sooo good leftover I would always make extra.
Definitely room temperature.