I have no experience with the raspberry pi as a cnc or printer. Ramps is much cheaper and more widely used. I looked into linuxcnc and it didn't fit my needs. The currently recommended software is cheap/free and easy. I need to be able to multitask while my cncs are running for hours on end and linux isn't an option for me at this point.
1- You can use home alarm cable, stranded, or cat5-6 cable.
2- 10-12" should be pretty good.