How do clouds know when to rain all at once? How do they all get full at the same time and rain all at once? What happens when one is not full enough? Would there just be a dry spot encompassed by rain?
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It only rains when the air can no longer suspend the water in a cloud. This normally occurs with a temperature change.
Air can be humid (it can hold moisture, water). How much, that depends on the temperature and the pressure of the air. In general, a cloud is “stable” in that it won’t empty out its water unless it hits an air current that has a different temperature or different pressure.
If you look at a [weather map](https://www.rochesterfirst.com/wp-content/uploads/sites/66/2020/04/Fronts.jpg?w=759) the rain front happens at the border where hot air meets cold air, or where high pressure meets low pressure. It doesn’t happen in the middle of the high pressure or low pressure areas (or high temperature / low temperature), it happens only at the “border” where there’s a shift from one to the other.
It’s kinda like soda, if you shake it (change the pressure) or you warm it up suddenly, it will fizz out. Water can hold a certain amount of carbon dioxide, and it depends on temperature and pressure. Air can hold a certain amount of water, and it depends on temperature and pressure; when conditions change (suddenly) the stuff comes out (as fizz or as rain).
To answer your specific question, it rains in a large area because clouds are rather large, and so a “rain front” can spread across several states worth of area / space.
It can also rain in a “small spot” like you envision, that’s typically called a tornado, where the change in pressure is concentrated and “very small” by geography standards and the tornado or “severe thunderstorm” will mess with the pressures and temperatures across a small area, relatively speaking.
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