How to simulate particle so bigger particles will fall faster than smaller ones In Blender, i have a particle system with random size of particle element. How can i simulate so the bigger particle will fall faster than smaller particles. The idea is to have different 'drag' effect to each particles where the bigger the particle, the smaller the 'drag' will affect it or in simple word : the bigger particle will easily fight air resistence so it will fall faster. Is it any parameter to set to handle this task ? 

So far i can think only to create multiple particle system with different 'drag' value then combine those particles systems.

edit :
additional notes...
 i'm not trying to simulate the ideal physical world, but real world. in ideal world v(speed) is not affected by mass, but by acceleration (a) or in this case gravity (g). v=v0+gt. and since the g is constant and no v0 (initial velocity) then the V will be the same -> in ideal world bigger and smaller fall the same, the only different is the energy need to stop them. So i want to simulate the real world where the bigger mass will 'easily' fight air resistance as its force (f=ma) is bigger so the air don't have enough energy to stop them. simple example : the real world like when bullet hits a wall and when debris is falling , the big chunk will fall faster (air can't hold it) while the smaller debris just need to float before hits the ground. The story will be different if there's no air.

Also what i mean by bigger particle is bigger in term of mass not just the volume. Bigger volume does not neccessarily have bigger mass. As in my example : bigger chunck of wall hit debris. Also i'm just curious if this can be done in blender simulation, i know for my case i can solve it with other way, but i'm just want to explore how far this blender sim can go.

Thanks
If you don't want a real world physics involved You can do that easily with Particle Texture.


Let assume that you want the particles to move downwards (-Z) emitted from a Plane.
Set the particle system Field Weights - > Gravity to 0
Set the direction of the particles to Normal : -3
Optionally, set the all the particles to be emitted from the Start frame
Set the size of the particles to some value. You don't need to randomize the size with the Random option at this point.
With the Emitter object selected go to Texture tab in Properties editor. Make sure you're in Particles Textures .
Create a new Texture.
Make the new texture to be of Type: Clouds and for Basis chose Cell Noise. Let Coordinates be Generated.
And finally in Influence deselect all and turn on Size in General and Velocity in Physics.


In this way the color of the texture will control the size and velocity of the particle by it's initial location. Lighter values will increase the size and velocity. Darker values will do the opposite.