The main variables in sizing a gutter system are:
In a storm event rain does not fall straight down but is blown. With blown rain, the steepness of the roof impacts the amount of water that needs to be drained. The steeper the roof, the more water the gutters will need to handle. Understanding the surface area of the roof, the steepness (or pitch of the roof) and the maximum rain level to be planned for as specified by the Internation Plumbing Code (for example Everett gutters need to handle 2.1\” of rain per hour) your gutter installer can determine how many gallons per hour the gutter system must carry.
Each manufactured gutter system including half round, box, and k-style gutters are specified for capacity. Capacity is also affected by the slope of the gutter. The higher the slope, the more capacity. Moving from a 1/16\” slope to a 1/2\” slope increases capacity by more than 2.5x Alternatively, more downspouts can be added to the system. Surface area drained increases with each additional downspout.
While it is possible to make all the trade-offs yourself, you might want to consider allowing a professional gutter installer plan out your system for you.