Here's a good example of a market sizing. How many people live in China? A billion. Because the population of China is young, a full 600 million of those inhabitants might be of child-bearing age. Half are women, so there are about 300 million Chinese women of childbearing age. Now, the average family size in China is restricted, so it might be 1.5 children, on average, per family. Let's say two-thirds of Chinese women have children. That means that there are about 200 million children in China. How many of those kids are under the age of two? About a tenth, or 20 million. So there are at least 20 million possible consumers of disposable diapers.

To summarize:

1 billion people x 60% childbearing age = 600,000,000 people

600,000,000 people x 1/2 are women = 300,000,000 women of childbearing age

300,000,000 women x 2/3 have children = 200,000,000 women with children

200,000,000 women x 1.5 children each = 300,000,000 children

300,000,000 children x 1/10 under age 2 = 30 million