hope you don't mind my stinky friend,
Sup Newb.
Water from the roots, CO2 in the air, and light combine together in the leaves to create the energy that drives the plant and puts out O2 as a byproduct. carbon dioxide + water + light energy → carbohydrate + oxygen. . . How fast does your plant do this?
Whenever you are growing something, it's growth rate is only as fast as what ever the greatest limiting factor is, or the weakest link in the chain so to speak.
For example, a plant can use up to x=1, y=1, and z=1, where x,y, and z are factors like amount of light, amount of nutes, maximum genetic metabolic rate, amount of CO2 in the air, etc. etc. all of the million factors.
so if you have x=.3, y=.4 and z=1 your plant is only going to grow at .3 of its maximum genetic metabolic rate, which is the true overall limiting factor.
ambient CO2 in the atmosphere is well below what it was in the past, and subsequently plants can grow a lot faster with enhanced CO2. So you can think of it like CO2 in the air is at x=.3 and by kicking it up to 1500 ppm you are kicking it up to x=1
now the beauty of this analogy is that it leads to another obvious conclusion.
pumping x up to x=1 is gonna do you no good if A, B, C, y or z = .6 or .7
so if you have a ton of light, and nail your nutrient levels, and max everything out, you are losing a lot of growth speed by not using co2. on th other hand there is no reason to pump up co2 if you only have a <400w HPS.
bottom line is you can use it to shave time off of your grow. . . there is conflicting evidence as to whether or not it can increase size and weight.
hope that makes sense.