I wonder where can I foundthe ArduPilot Mega ATMega2560 board PCB design (Dimension)
Because I am designing a frame based on the GAui 500x and I would like to integrated this PCB board in my drawings.
I look for it in the WIKI but couldn't find it
I don't believe a standalone mechanical drawing exists. The best place to grab dimensions from would the EAGLE .brd file, available in a .zip on the product page.
You'll have to download and install EAGLE.