I'm with ic-racer--contrast often has to be increased as print size increases. You may not notice it from 5x7 to 8x10, but 8x10 to 16x20, it's noticable. Sometimes it could just be because you're changing the way you view the print, and you might need more contrast for the image to be effective from a greater viewing distance. It's fairly common for me at least to print large at a grade higher than I print small for the same image.

To get a ballpark exposure time, figure one stop for every standard print size. You'll notice they follow the f:stop series approximately: 4x5, 5x7, 8x10, 11x14, 16x20, 20x24.