We can call the growing droplet a raindrop as soon as it reaches the size of 0.5mm in diameter or bigger. If it gets any larger than 4 millimeters, however, it will usually split into two separate drops. The raindrop will continue falling until it reaches the ground. When the drops finally reach the ground, the biggest drops will be the ones that bumped into and coalesced with the most droplets. The smaller drops are the ones that didn't run into as many droplets.
Raindrops are different sizes for two primary reasons.