In firmware, the printable area of delta printer is set to be a circular area. But you can’t set the printing area diameter as large as the printer head reachable area (yellow circle), because the effector and fan need extra space. You have to set a smaller diameter (blue circle) to avoid impacting the belt. (Image originates from reprap)
How can I make use of most of the printer head reachable area as printable area? Slicer software e.g. Slic3r/PrusaSlicer has feature to set bed shape from a STL file. It allows you to set bed shape in any irregular shape, and it will alert you if the part is placed outside the printable area. In this way you can set a large printing area diameter in printer firmware, and exclude the areas where interfering will happen inside slicer bed shape setting.
I used Fusion 360 to model the real printable area of my Kossel Mini printer. The print bed glass is 200mm diameter, so I first drew a 190mm diameter circle as the basic printable area. It’s also set into the firmware.
I connected the printing host software on PC to my printer and sent G-code to find out the edge positions of the printable area.