Estimating market sizes
nostrademons: They're very often wild-assed guesses.  The introduction of a revolutionary new product often changes the market size significantly.  For example, the market for computers got a lot bigger when the Altair was introduced (a world market for 6 computers?  Really IBM?), then bigger still when the IBM PC was introduced, then bigger still Compaq and Dell and clones came about, then bigger still when Win95 provided the first decent GUI for clones.You can make some rough estimates by figuring out what substitutes your product has, and then researching the combined revenues of all the products you'll be replacing.  This also degenerates nicely: if you can't think of any substitutes, then it's fairly likely that you have a $0 market.