Today's wide variety of rig designs makes it difficult to develop a general rule. For example, a 70's style boat will frequently have a small main and require a large genoa in order to develop sufficient power to drive the boat. Conversely, a common trend in recent years has been the use of fractional rigs with large mains and smaller jibs to power the boat. (The idea being that a main is easier to manage than a headsail owning to the fact that the boom captures one side.) This means the boat's sail design can determine the choice between genoa and jib. After boat design, the variables come down to the wind conditions and sailing style you anticipate. Simplified, you could say that sailing in predominantly light winds would most likely lead you to a larger genoa. Moderate winds would probably dictate a smaller genoa or a jib. Heavier winds will push you towards a jib. Within each size range, you will also make choices about construction and materials based upon the same factors of anticipated winds and your sailing goals. Your sailmaker should work with you to define those requirements so the two of you can factor them into the design choices.