A Hard Rain Is Falling

In a vacuum, everything falling to earth accelerates at 9.8 m/s^2 (32ft/s^2) [the downward speed increases at 9.8 m/s (32 ft/s) every second.]

Rain accelerates down to earth initially at 9.8 m/s^2, but air resistance slows the drops to a constant speed called the terminal velocity which depends on the size of the drop. "When all the parameters are considered the terminal velocity of a typical raindrop is calculated to be about 9 meters per second or 20 mph. A smaller raindrop of radius 0.15 cm has a terminal velocity of about 7 meters per second or 16 mph." (1)

Velocity is only one factor in calculating the momentum of rain. The mass of rain also factors into the momentum.
p = mv
p = momentum
m = mass
v = velocity

"Climate change can affect the intensity and frequency of precipitation. Warmer oceans increase the amount of water that evaporates into the air. When more moisture-laden air moves over land or converges into a storm system, it can produce more intense precipitation -- for example, heavier rain and snow storms." (2)

Part of the increasing momentum is transferred to the sides and upward increasing wind turbulence, as well as updrafts.

Most of the momentum is transferred upon impact. You may notice the rain bouncing higher off the streets and sidewalks. As rain becomes more massive, it will have greater momentum when it hits the ground. Ground without groundcover will be hit harder causing more damage. The groundcover will also be hit harder causing more damage.
* Concrete and asphalt will sustain more damage.
* Solar panels and roofs will sustain more damage.
* Trees and plants will sustain more damage.
* Food crops will sustain more damage.

I asked Sidd: Is there a limiting factor to the size of raindrops... that will prevent them from becoming lethal?

Sidd replied: if it freezes, yes, it can kill you.

I imagine around a centimeter is about as big as raindrops can get... would break up bigger than that for sure.

Never heard of raindrops killing anyone, but hail, yes.

A week later it was reported, Giant hailstones kill toddler and injure 50 people in Spanish town
A violent hailstorm has lead to the death of a little girl in Catalonia -- "Hailstones as large as 10cm (4inches) fell over 10-minutes killing the 20-month old girl."