If you adjust the f/stop to give the same light level, the exposure time and contrast should remain the same. Going from 8X10 to 16X20 requires opening up two stops. The required exposure is proportional to the area of the print. If you have to increase the exposure time, you may have to adjust the exposure for reciprocity. How much depends on the paper, and none may be needed. You can check this by making a small test print from a portion of the larger sized image. Dodge and burn times will increase in proportion to the increase in overall exposure time. Contrast usually doesn't change with exposure, but it may. You just have to check. Some lenses, enlargers, and darkrooms (due to light splatter) have enough flare so that contrast gets reduced when making big enlargements. You just have to experiment to tell if it will happen to you and how much correction is needed. It may vary from negative to negative, also. It helps to be careful about flare and to try to eliminate bright reflecting surfaces near the easel. Once again, you can usually tell what corrections are needed by making a small print of a portion of the large image.
Originally Posted by tkamiya