I want to convert fix wing plane to VTOL but I don’t know the conceptual design of quad arms motors.
Please advise me, How to design the appropriate arm length and installation position of quad motor arms on fix wing plane.
Make sure the props won’t hit anything, make sure the arms are securely fastened to the wings, and ideally try to keep the center of gravity of the aircraft near the middle of the four VTOL motors.
If you google “quadplane ardupilot” and select the “images” tab, you can see lots of examples.
For a quadrotor you want roughly square frame though I suspect going wider could improve hover roll authority (compensate for larger roll inertia) other than that keep motors centered around CoM (about 25% mean aerodynamic chord).
If you don’t plan to do extended hovers you can go for higher prop loading than typical copters, it will be less efficient (for a short time) but should slightly reduce drag (affecting most of the flight).
Keep TWR above 2 for controllability see Vortex Ring State discussion for why.
Make sure your airspeed sensor is set up correctly it makes transition significantly more reliable.