Here's the one I made a couple of years ago and am still using (but need to update too!):
It's actually a small card (7" x 7") that I prop up inside the lower right windshield. I prefer to use something like this because I think bigger signs detract from a display, esp. if they prevent getting a full view of the car, they can be hazardous if they get loose and, as it was pointed out, very few people bother to read much of what's on them, if at all anyway. But it's still nice to give some basic information as to what exactly it is that you're showing for the truly interested folks.