When we go to beach, we feel cold on a very hot day, that's the basic principle of evaporation of water on the surface and takes the latten heat out from surrounding and lowers the temperature due to evaporation. Same like if we put a drop of water in hand as the water evaporates we feel cold. The Evaporative cooler work on same principle as we use wetted filter pad to evaporate water and fan to force the air circulation. These devices cool by evaporating water. A fan draws warm air from the outside through a series of wet filter pads. The air's heat evaporates the water, cooling and humidifying the air, which is then blown into the house. It's important to ensure good ventilation so the humid air doesn't accumulate inside.

Industrial Evaporative Cooler

The higher the outside humidity, the less efficiently evaporative coolers work, as the humid air from outside won't be able to evaporate much more water from the filter pads.

Size doesn't really matter. Of course, a larger, more powerful air cooler will have a stronger air flow and be able to blow cool air over a larger area; but essentially they don't need to be matched to the size of the area like air conditioners do.

Unlike air conditioners, you don't have to seal the room or house. In fact, you need to keep a couple of windows or a window and door open because the cooler needs this air flow: it sits in front of an open window or external door and draws the outside air through it.

Industrial Evaporative Cooler