To print a good quality poster 20 inches wide, 30 inches deep at 200 DPI we would need:
20 x 200 = 4,000 pixels wide
30 x 200 = 6,000 pixels high
4,000 times 6,000 = 24,000,000 or 24 mega pixels.
As long as your camera is taking pictures over 15 mega pixels you are recording more than enough information for a reasonable print at any size. You can use software to enlarge your image suitable for printing at huge sizes - even billboards.