Unfortunately, it is not really that simple. There is not an equation where you just plug in the variables.
Your mostly going to be constrained by fabrication and dollar constraints. If you are building this yourself to save money, what you are able to build is key. Coils are nice (like a big wort chiller), but tough too make in a reasonable size.
A staggered tube arrangement would be easier to build, but takes a lot of brazed or soldered joints.
Is your reservoir round or rectangular? How big is the space in which to insert the coil, or is the top open?
Since this is going to be inline with your glycol pump feeding your FV's, you want to keep pressure drop low. You can do this by putting your coils, or staggered tubes in parrallel.
Another angle is to check out cryogel.com. Cool product. No pun intended.
Ice storage will have the same effect as a large reservoir. The energy storage is just denser (144 times) because of the latent heat of fusion of ice over water.
What is fairly easy to calculate, is the size of reservoir required to keep the temp rise within a given range. But this could be moot since you already have a reservoir you want to use.
Even if you are good at brazing, I would just solder the joints to to the speed of soldering over brazing.