With the DBT FAPC, you decide your size based on your plate size. The medium and the large are actually the same size, because they fit the same sized plates (10" x 12" plates), meaning they cover the same amount of area on your chest. The only difference being in the shoulder straps. The large has longer straps than the medium. I'm 5'11 around 180lbs w/ athletic build and I had a medium size, and I had the shoulder straps on the shortest position. Considering the fact that most people use their PC's entirely too low on their body, I'd recommend getting a medium because you can elongate the PC quite a bit with the straps. You may want to call DBT as well to find out what they recommend.
Another thing to keep in mind, is that the FAPC has JUST enough material to cover the plate, therefore the sizes tend to run slightly small. For reference, cut out some cardboard or paper in the plate size, and add maybe an inch to every side, and that's about how much coverage you're going to get with the FAPC. It is designed to be a normal plate carrier, very much like what you see with regular vests that only take plates, with the exception that they added molle webbing to allow carrying pouches if desired.
Continuing with the fact that most people run their PC's too low, if you ran the FAPC correctly, you'd have MORE than enough room for a battle belt underneath it, allowing for more molle real estate if needed.
It is not at all uncomfortable to run a PACA underneath it as I did in my pictures. If anything, it secures the FAPC even more against your body. Be aware that it WILL get hot in summer conditions and you will suffer from heatstroke if not properly hydrating. If you are rather heavy for your height, then it may get a little bulky, but it's still manageable considering it's a lower-profile(d) rig to begin with.
Hope this helps.