How to size air conditioning outlets(grilles)?

Duct systems are not self-balancing. The closest thing to a self-balancing duct system would be one with an adequate return in every room that has a supply outlet. This isn't very practical, and really can't be achieved in the real world. Installed ducts rarely perform like they do on paper; they must be adjusted individually after installation, using proper testing and balancing methods and accurate instruments.

For example, a 20-in. x 25-in. grille has 500 square inches of gross area (500 x 2 cfm= 1,000 cfm). Using this approach, two 20-in. x 25-in. return grilles will be the right choice for a five-ton HVAC system in order to actually deliver 2,000 cfm back to the blower without causing the return to hemorrhage.

hello 

to size a linear grill you must take care about many parameters 

1-CFM per foot 

2-Listed height 

3-friction losses 

4-face velocity 

5-sill and sidewall throw

6-minimum ceiling height 

7-NC

lets imagine that you have a concealed A/C with 1000 CFM connected to only one  sidewall grill 

1-CFM per foot : if the standard length for a sidewall grill is 4' , so you have now 250 CFM/ft , that will lead you to all parameters remaining- from 250CFM/ft you will find that available listed height is 6 , 8 and 10 inches

2-Listed height : so you now have 3 options 6 , 8 and 10 inches , your decision will be according to selected depth of your duct ( you sized the duct according to recommended velocity referred by SMACNA- if this FCU is located in residential application so you can do you calculation in velocity 800 FPM- after sizing you can obtain that the duct depth will be 10'' )  

3-friction losses : now after you have CFM/ft and listed height you can easily find that friction losses will be 0.01 InWg  ( you should accept this value or refused - check blower performance vs external static pressure in A/C catalog )

4-face velocity : and also you will find face velocity 550 FPM ( accept or refused according to SMACNA recommendation )

5-trow : you will find that sidewall trow range from 21 to 32 ft ( accept or refused according to room or space dimensions ) 

6-ceiling height : the ceiling height in this case must me minimum 15 ft 

7-NC : and NC here is 25 ( i would accept 25 NC for residential applications )

all these data from ZamZam ( linear grill)  airoutlets catalog you can find it in internet 

Good luck 

Outlets ( grills, registers , diffusers) are sized based on  volume of air (in CFM) to be supplied to the space, velocity of the supply air and noise level. smaller outlets will generate noise