Newbie quesitons before build my own custom ARDUINO MEGA 2560

"Download free version of EAGLE
PCB design software for students, makers, and professionals.
Includes 2 schematic sheets, 2 signal layers, and 80 cm2 board area.
Available for Windows, Mac, and Linux."

You can get a lot on 80 cm2.
A full size Mega is 10.1 x 5.35cm, or 54 cm2.