What should it be to get good gas milege?

I have 225/50/16 tires and I keep my pressure at 36psi in the front minimum in the summer and around 38psi in the back. I like to have more psi in the back.

Peace.

Nissan Specs - 32F 29R

Does a different tire size change the recommended tire pressure?

I use 35 all around (maximum pressure for my tire). There is a decrease in ride comfort, but I've observed an increase in overall handling, better fuel economy, and more even treadwear. Personally, I think Nissan's recommendation is a compromise between ride comfort and handling. So it comes down to a personal preference based on what your main desires are.
BTW, a police pursuit driving course I've taken teaches to use higher pressures for better overall traction. Higher pressures make the sidewalls stiffer. Some argue that it decreases the footprint, but I think that stiffer sidewalls makes up the difference. And going with the decreased footprint argument, the higher pressures will improve traction in rain/snow because the "thinner" tire will slice through it easier, so less hydroplaning (ever wonder why sports cars with really wide tires have terrible all weather capability, besides too much HP? Wider tires "float" across water/snow).
I'd suggest you experiment and find the ride you are after. Keep in mind to check your tire pressures cold (morning is great) for accurate, consistant readings.

Dave