My CFC is 25' of 3/8" od tubing . If I'm gravity draining, the flow is very slow and I can't run tap water through the chiller slow enough for it to get even luke warm. You will probably not see a major increase of temperature in your reservoir and will probably not even need 10 gallons.
What you're mentioning is a super efficient way of chilling, a you may actually over chill.
If you are gravity draining and haven't build your chiller yet, I'd suggest going away from the widely accepted 3/8" OD copper and upping to 1/2" for better flow. You will probably not need 25', as is generally suggested here, either.