At 300ppi, a 16" by 20" will require about 32 megapixels. But for such a large print, you may get away with 200ppi, which equates to around 15mp, which most cameras are capable nowadays. Well exposed or not depends on lighting conditions and photographer's skills.
You should be able to. I enlarge a 21mp image to AO size on screen. It looked pixelated on screen but when you print it out and even on close look, the image looks almost perfect to the naked eye. I printed using a large format inkjet printer - one that was recently launched by Fuji Xerox.
I got this done on 16 x 20 canvas through Snapfish awhile ago. The first link is the actual file I sent to Snapfish from flickr. It is from a Canon S90. It came out pretty well. I guess if you are wanting fine lines of buildings or landscapes etc then you may notice some decrease in image quality at that size, but for the photo I printed, it's pretty good. http://www.flickr.com/photos/ozgfk/...photolist-9BWPxA-9BDgpf-9BDj8y-9BDr1S-9CgWKm/