The cooling water or cooling air determine the temperature and pressure of the condenser. The volumetric flow rate at that temperature and pressure determine the size.

I just noted the rest of your question. Most commonly the cooling water or air temperature fixes the condenser temperature for power-only systems, meaning that the exit steam pressure and hence the plant efficiency depend on weather and season. Combined heat and power systems are different. Commonly, the hot water has a higher priority than power generation, so the amount of hot water/steam generated follows a dispatch schedule and the power generation changes accordingly.

I agree with @ Larry L. Baxter

Hi Larry

So you are saying that for an air cooled condenser, the fan speed/#of fans/air flow rate will mostly stay the same? and the exhaust steam pressure and temp. will simply follow ambient conditions?

Thanks

Generally, that is correct for a power system, with some variability. For example, the system will avoid freezing the steam/water in very cold weather and will probably have a cold-temperature limit. It will not generally have a hot-temperature limit. Combined heat and power systems vary the condenser active cooling varies much more with conditions since they generally try to produce a dispatched amount of steam/water at specified conditions.

Thank you so much Larry