Rain drops are often drawn in books or shown on TV as being tear drop shaped. This isn’t true at all. In reality, the shape of a rain drop changes as its size gets bigger, and they are never shaped like a tear drop at any point during their life-cycle. They are actually spherically shaped when small and look more like a jelly-bean when they get larger. Their shape is a result of two things; water surface tension and wind resistance.
An example of water surface tension would be the shape water takes as it collects on your car just after it has been washed and waxed. The water globs up and sits on top of the car instead of spreading out into a thin layer. Wind resistance on the other hand, creates a force on the rain drop which causes them to deform and break into smaller droplets. Imagine blowing on a large bubble and noticing the change in its shape. When the wind resistance overpowers the water tension force, the water droplet deforms so much that it splits into two smaller droplets.
Water tension and wind resistance therefore are constantly affecting the shape of a rain drop as it falls through the air from the clouds high above. The water tension keeps the water droplet intact and is stronger then the wind resistance that is trying to break it apart. However, as the rain drops combine, the surface area increases and the wind resistance starts to affect its shape even more.
When the rain drop is small in size, about 1 mm in diameter, it take the shape of a small sphere and is virtually unaffected by the wind resistance. At 2mm, the rain drop is still held together by the water tension but it starts to get deformed by the wind resistance. As a result, the bottom starts to flatten out a bit. At 3mm, the rain drop looks more like a jelly bean than a sphere, but the water tension still holds it together. At 4mm, the rain drop becomes very deformed and begins to look like it could split in two. At about 4.5mm, the water tension is over powered by the wind resistance. The rain drop is so deformed it creates a large canopy (almost like a parachute) and splits into two or more smaller drops.
During a very heavy rain, these rain drops can break apart and re-combine into large drops at quick rate. So for some of their life time, they can exceed 4.5mm in diameter. Also, if the rain is partially frozen (eg; melting hail or sleet), the size can be slightly larger and the shape more irregular.