How big is a market when none exists yet?
When looking at the white space of a category for new opportunities the wrong question to ask is: "How big is the market?"
By definition new areas have no defined market boundaries so the answer is 'zero'. This is hard for executives to hear - they want safety in numbers. Sure, we can come up with numbers but latent demand, like the advertising value of UofP's exposure on the super bowl, is a 'bit' difficult to calculate and defend. Any answer is wrong - just remember the need for 5 computers and 900,000 cell phones.
The objective of analyzing white space in a product positioning exercise is to determine if it is probable that a category could be created. Is there a need , or more often a combination of needs, that can be satisfied totally differently than alternative offerings? Note that most new markets tend to be a subset or reconfiguration of existing, larger markets. Consider that Billboard now tracks 45 charts for singles, 47 for albums and another 7 for videos. The second question is: Are we capable of satisfying that need? If you can articulate a need and envision a solution then you're on the right track.
The goal is very different than a line extension exercise.