Simply name the market segment that this product falls into. If you cannot name the market segment, then you probably do not know enough about the overall market yet. If your product bridges two market segments, list them both. If your product is so novel that it does not fit any existing market segment, name a new market segment using a combination of terms that describe current segments, or choose the market segment that your potential customers will think that your product falls into (until the learn more about it).
Estimate the size of the population of potential customers in terms of number of customers or total dollars. Estimate the entire market, including customers currently served by competitors, but do not count customers who could never buy your product.