My diptube is off to the side and I plan on using a 5 gallon corny keg to wrap the IC. The diameter should be 9-10 inches. The copper tubing is 1/2 x 50"
50 ft should be fine, I have a 50 ft I made and can cool 10+ gallons in 20-25 mins. I also soldered legs on it so it sits over my dip tube, I also wrapped mine around a corni keg to mould it.