As a rough guide the AXA is for up to 12'' swing, the BXA for 12-15, the CXA I don't remember but it would be rather big for a 12'' lathe.
It really depends on the space available on your particular lathe. On my 12x36 (typical) lathe the BXA holder with 1/2'' tools is just right.
But check the critical dimensions of your lathe and those of any QCTP size you would buy to visualize beforehand where the holder would ride in the tool post when the tool is on centre.