Hi, your question was a bit ...

Hi, your question was a bit open ended and I wonder if some of the other answers may have missed the mark. 

Yes, you can start with the water analysis and calculate loading (the equivalent concentration of cations and anions in the feed that need to be removed).  You also need to calculate the operating capacity of each resin used in the mixed bed (which is a fraction of the resins total capacity depending on degree of regeneration and the mix of ions in the feed.  Once you have done this, the volume calculation for the resin is straightforward.

Resin volume equals throughput times resin capacity divided by loading (or if you prefer, resin volume times resin capacity divided by loading equals throughput).

You can request a resin rating from various resin companies, some have on line software that you can use or there are older charts and such that provide capacity information in hard copy formats.