Regardless of copper pipe size cold fall is gonna flow downwards, it is more dense than warm air - which is why I put a fan in to blow the cold air up. The main purpose of the copper is to chill the beer lines through conduction, so having those beer lines pressed up against the tubing would be best, so smallest copper pipe you can stuff your beer lines into is the way to go.
You could try a separate copper pipe for each line if you wish but I figure that the pair of lines in one large copper pipe has more surface area in contact.
What's on tap... I don't even keep track anymore.
Reason: Made a science typo... cold air is MORE dense... not less.