How to size a correct throttle body for their engine? This TB will control the fuel (NG) line, not on the mixer line. What information do I need from them and how do I calculate the size of the TB?
Is there any relationship between the Cv data and the throttle flow area?
Therefore a Holley 650 flows 650 CFM  with atmospheric pressure at the inlet and 1" Hg subatmospheric down. 
Arbitrarily selecting a 345.6 cubic inch engine for simplified math (I hope): it will theoretically inhale 0.1 cubic feet of air per revolution.  ( there's the simplified math part)
If you spin it up to 4000 rpm, there's a 400 cfm requirement.(Assuming volumetric efficiency of 1.0, which is unlikely unless using tuned intake)  
THat would require a butterfly-valve with a Cv of 157, which would be really close to a 3" diameter throttle plate. 
Put in your motor parameters and ratio to this. 
Throttle Cv would be ratioed, also. 
Cv is proprtional to the AREA of the plate, so
d=SQRT(Cv/17.4)

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