I use 11x14 inch paper for contact prints. That is large enough to be sufficient for any format negatives which I use. If I have much wasted space on the paper, I cover it with mat board cutouts so that I can write notes with a marker in the margins.
The number of negatives actually printed varies depending on subject matter, quality, and time. I shoot mostly 4x5 and print at a ratio of 1:4 - 1:10 most of the time.
Although this is slight heresy, I don't like to make contact prints. Too much to keep track of, too much time, etc. If I do make contact prints, I buy the cheapest 11x14 Arista RC paper and crank them out. Otherwise, I like to study large format necatives on a light box with a loupe and try to figure them out without a positive. Another form of pre-visualization I guess, like reading an x-ray. Of course, 35mm negative are another story and I would make contact prints because they are so small.