Evaporative coolers lose their effectiveness in humid areas. But they will also perform poorly if the windows (or other openings) are open too far, or if they aren't open far enough. Windows (or other openings) are critical for the performance of evaporative coolers.
Their effectiveness depends also on proper sizing, and, in a lesser degree, on features like thermostats, filters and the number of speeds. A vent-only-option can also be important for energy-efficiency strategies.
It’s a well known fact that evaporative coolers are useless in humid climates. But ventilation is also crucial for a good performance. They cool by moving air (and by using water to cool the indoor air); to function properly, inside warm air has to be exhausted out of the home.
Portable evaporative coolers, with their very low capacity, may not need significant amounts of ventilation. They may even run without it, at least for short periods, in homes with low levels of insulation and low airtightness (see the manufacturer’s literature on that). But window and central evaporative coolers depend largely on open windows or vents.
Obviously, evaporative-window units do not need windows or other openings to draw in outside air (they capture it directly), but they require them to exhaust air to the outdoors.
As to central units - with the exception of sophisticated ducted systems with dedicated openings -, they need windows or vents on the upwind and downwind side of the house.
Evaporative coolers require an opening area (given by windows or other openings) of about 1-2 square feet on the windward side of the house, for each 1,000 CFM of cooling capacity; if your cooler has a capacity of 5,000 CFM, you will need an opening area of about 5-10 sq feet.... And you will need an equivalent opening area in the downwind side of the house.
Make some experiments with the windows to open or close, and how much. You can choose which rooms to cool by opening windows in them. Closing all the windows of a room will concentrate the cooling in the others, where you want it most… Make some experiments, looking for the best results.
But do not forget: if windows are open too far, unwanted hot outdoor air will enter into your home; if the windows aren't open far enough, the system will create too much humidity, which may reduce your comfort.
If shopping for an evaporative cooler, if you live in a climate with cool evenings and nights, make sure the evaporative cooler has a vent-only option. It will allow you to run the evaporative cooler as if it were a whole-house fan (or a powerful window fan, in the case of window evaporative coolers).
In other words: the vent-only option transforms the evaporative cooler into a powerful ventilation fan able to capture breezes and to cool your home in some few minutes, while also exhausting the indoor air. You just have to open the windows on opposite sides of the house, and let the equipment do the rest, quickly and at a very low cost (the cooler will consume the equivalent to a lighting bulb, often less than 100 watts).
Rules involving the sizing of evaporative cooling
There isn't sizing rules for portable coolers. They are low-capacity units, typically bellow the 600-700 CFM, often unable to provide enough cooling in very hot weather conditions.
As to the sizing rules for other coolers, you don't have to do accurate calculations.
You just have to multiply the surface of the area you want to cool (the area of the house...) by the height of the rooms (often 8 feet) and divide the result by 2. An example: for a 1,000 sq feet house, choose an evaporative cooler with 1,000 x 8/2 = 4,000 CFM.
Though it doesn't involve sophisticated rules, sizing is important. A too large unit will create too much humidity; an undersized unit will not be able to cool effectively.
Other features that matter
Choose an evaporative cooler with a thermostat. That’s almost standard now. A thermostat can help you to reduce the energy and water consumption, and will reduce the risk of overcooling.
Filters are also important: they can help reduce the dust, mold spores or pollen, a major feature for people concerned with respiratory problems.
Also prefer a unit with at least two speeds. It adds flexibility to the cooler and provides a better temperature control, which is important for energy savings.