There are a lot of methods, approaches on how to calculate the market sizing of a niche industry. The most common methods are top-bottom, bottom-up, comparable markets/replacement, new market approaches.
The critical step is to identify the key factors and logical tree of Your calculations and assumptions (e.g. for diapers market: a) # of babies consuming diapers in a given day b) # of diapers baby uses per day c) average # of days per year.
There are a lot of statistics and information sources to assess key factors.
Another way to estimate market size is to conduct own survey (for a targeted market and potential users) to get the answers to all questions and check Your assumptions.