Paduk,
If you have electric boilers you can disregard the following. But if your boilers are gas fired, please read on:
Is the reason you want a buffer tank is to keep your boiler(s) from excessive cycling? Do you have gas fired copper finned tube boiler(s)? If so, copper finned boilers are notorious for requiring a buffer tank because of the low water volume in the boiler. Without additional water volume the copper finned boiler will cycle on and off every few minutes. Is your system serving 2 way control valves? 2 way valves make the cycling even worse at low load. Typically copper finned boilers require primary-secondary piping when serving 2 way valves. On low system load requirements the boilers will cycle off on high limit because the heat has nowhere to go but back to the boilers. Is the boiler control single stage (on-off), two stage (high-low), or more? Typically copper finned boilers can be as high as 4 stages of fire.
The amount of buffer tank volume to keep the boilers from cycling is drastically different from the hot water storage system tank volumes calculated above. Depending on the number of boilers and the number of stages of fire of those boilers, the tank size can be reduced significantly. A tank as small as a few hundred gallons will reduce the cycling of a boiler plant significantly because you only have to provide enough volume to correspond with the lowest fire stage of the boiler plant. Example: If you have a boiler plant that is a total of (3) 2,000,000 BTUH output boilers and each boiler is a 4 stage boiler, then the buffer tank only has to be sized for 500,000 BTUH (the lowest fire situation). Then all you have to decide is how long you want the boilers off for in between cycles (10-15-20-30 minutes, whatever) and size your tank accordingly.
I have attached a pdf for a buffer tank that you may find helpful if you are working with copper finned (or any other low water volume) boilers. It walks you through the entire sizing procedure. Good luck.