Back in 95, when I bought a new Coleman Popup, they stopped offering the smaller Mini-Mach A/C unit in Florida as it couldn't keep up with the heat load. The only model offered was the 13,500 BTU model. That's what we had on ours.
If camping in the South, with no shade, it's still not enough A/C to keep up with the heat coming in through the tenting above the beds. I bought some reflective blankets at Sportsman's Guide, and attached them to the bunks with clothes pins. That made a huge improvement.
Have fun!