D.B.
I think people take far less, especially in a buffet line, than you would think - maybe 1/2 cup per person. There's only a certain amount of space on a paper plate.
How much lettuce you buy depends on what else you are putting in it. I'd make a salad for the family with one Romaine heart, and whatever add-ins you want, and then literally put some on a plate and see how many times you can do that!
You can put the salad in anything for the shower but if you want a fancier bowl, put half the salad in there and the rest in a tupperware container or even a big ziploc bag. If the main bowl gets emptied, refill it.
I'd also dress the salad just before serving, and I'd put in a pair of tongs vs. 2 servers. Nothing slows down a buffet line faster than people having to put their plate down in order to use both hands on the serving pieces.
Have fun!