The size of AC unit you would need all comes back to the science and lots of details needed.
What volume space are you talking about, what are the ambient temps throughout the year and day/night differrentials, how much heat do the lights generate, will the ballasts be housed in the same space, thereby increasing the temps. These are just off the top of my head but there are other factors too.
From what I have seen a 12ton unit will cope with most domestic situations of growing, remembering you should never run equipment such as an ac unit on full power all the time, there should be room to spare. Also, what is good for somewhere such as where I live (Scotland) will be different from Florida for example.
You really need to do a lot more research before spending money on an AC unit and CO2 ammendments and as I said previously, for 2 or 3 lights I just dont see the point until you have ran several grows in the space and got the space totally dialled in. Only once this happens can you quantify the value of adding expensive AC & CO2.
Heres an example of what I am talking about, a dialled in room can produce say 1g/W, a high tec room with ac & CO2 and top end nutes with all the add ons will produce, for arguments sake, 1.5g/w. Lets say you have 2x1000W. Room a costs $150 per month, room b $250. Now do the math,
a 2000g/$150 = 13g/$
b 3000g/$250 = 12g/$
My friend spends £30 per month on CO2 and I call him an idiot, for the same cost he could run an extra 4x 1000W lights. He doesnt understand my argument, he just sees the bigger yield. Maybe a degree in business management is my problem that I understand getting value from each £or$.
The downside, for those who cant see it, is this is all dependant on maximising yield per $, not yield per m2. CO2 with AC will give more yield for the available space when set up correctly but there are a lot of factors to take into account.
In short, get a 12T BTU unit, CO2 generator (not bottles) and computer monitoring/control. If I had unlimited space & £ its what I would do.
Peace W