I kind of doubt they will come out with a 19 or 20 inch anytime soon or if ever. It seems that a more compact version of the MBP is in more demand than a larger one. People who want a smaller MBP are complaining about the size of the 15 inch so larger than 17 inches would probably not sell in enough quantity for Apple.
I personally would never buy a 19/20 inch MBP because I like my laptop to still be portable. For me the 15 inch is just portable enough for my liking. I already know I don't like the 17 inch. A 19 is more like a hybrid between desktop and laptop which seems to be more like a "slim desktop". It doesn't seem that many people are in that market. I could be wrong though but it's definitely not for me.