Rain reaches a terminal velocity as it is slowed by aerodynamic drag. Large drops fall faster than small droplets, catching and absorbing them until the aerodynamic forces become too great and blow them apart again. This is illustrated in this link: http://www.shorstmeyer.com/wxfaqs/float/dropdeform.html
The largest stable drops are about 5mm in diameter and travel at 9m/s, though larger drops can exist for a period of time before they disintegrate. Here's a table of drop diameters and velocities. According to this source typically 4% of rain drops are 5mm with the vast majority (63%) being in the range 0.85mm to 3.2mm.
Interestingly the dynamic pressure on the 5mm rain drop at terminal velocity may be calculated to be about 50Pa. Equivalent to just over 5mm hydrostatic head (but acting over perhaps more than 1 second).