The case of the Extra Hearts:
Some creatures, like the octopus, already have multiple hearts. In the case of the octopus, this is to compensate for hemocyanin, which is less efficient than hemoglobin at carrying oxygen.
A cockroach was once thought to have 13 hearts, but scientists now understand that is a 13-chambered heart that operates fairly differently than ours. It runs sequentially and the individual parts can compensate for each other, making their hearts more resilient.
Perhaps an even better model is the hagfish, with 5 hearts (one central and four peripheral). A hagfish heart will keep pumping up to 36 hours in an anoxic environment (the kind of place hagfish tend to live).
Heart design is going to be pretty conserved evolutionarily, so you should have an idea WHY all these separate hearts evolved. It sounds like your vision is that the creature needs them because of great size. But blue whales only have and need one heart, so scale alone isn't necessarily an issue. Do these hearts attach to separate lungs/gills? Is each limb it's own independent circulatory system?
The size of your creature shouldn't be a factor with multiple hearts unless you want it to be. Mice have hearts similar in design to an elephant. Biology is often a case of a pre-existing structure evolving to a new function whether it is better for it or not. The mammalian eye, for example, had to re-evolve a different way of perceiving color than, say, birds, because our deep ancestors lost the ability to see color. But while it is hard to prove, the barosaurus may have had 8 hearts, so if you want size to be the reason, you can certainly justify it. The blue whale has a 400 lb. heart, thought to be at the outer limits of heart function. Getting much bigger than that would likely require changes in the circulatory system (like extra hearts...?).
As for the exact size of the heart, that depends on many additional factors. Mass of the creature and demand are going to be the biggest factors. Find a creature with a similar evolutionary niche and divide the mass of the heart by the mass of the tissue supported. (this part is similar to the answer given by user65535, who answered it before I did). The hearts may be smaller than this in a combined circulatory system, since the central heart will be doing more of the work, so exact details depend greatly on biology and function.
But you will want a reason for multiple hearts if you desire to be true to biology. This reason can, however, be something that existed in an ancestor of your current species.