Hi! Thanks for posting a question to such important topic: market sizing! I am not aware of good resources but am happy to provide a general approach and a practice example, followed by a discussion. Let's go for it:

A. Approach

Most market estimations in case studies will be "top-down": you start with high level figures (e.g. age, income) and segment them based on a clear rationale.

Overall, there are three main steps to estimate the size of a market:

1. Rationale

2. Assumptions and calculations

3. Conclusions and next steps

It is important they take place in the right order (1->3). Let's discuss what happens at each stage:

1. Rationale

This is the "theoretical" part, where you outline which drivers you will be looking at, and WHY your selected them (hypotheses). You can think of it as a "mini-framework": you will pick 2-3 drivers (imagine 2-3 "boxes"), and communicate them as you would in the opening of a case: "In order to estimate the size of the X market, I want to look at 3 areas")

You will then deep dive on each driver (without any numbers yet), explaining why each driver is important. In other words, you will state hypothesis outlining how each driver impacts the size of your market, such as the likelihood of someone buying a product). A hypothesis looks at the effect of variable A (independent variable, such as age and income) on variable B (dependent variable, such as the likelihood of buying a product). This should become clearer in the example.

Outlining your rationale upfront is very important. When you come in with numbers your interviewer will already know what to expect and will be able to follow you more easily.

2. Assumptions and calculations

Here you state the assumptions and perform calculations. An assumption describes the value of one variable, based on a certain premises (usually common knowledge). For example, we can assume the average life expectancy in the United States is 80 years, since we know the US is a developed country, its population pyramid is more or less constricted (you don't need to describe why you think the life expectancy in the US is 80 on an interview but I am making the point as not everyone is familiar with the difference between hypothesis and assumption).

3. Conclusions and next steps

Very important step. "Sense check" your numbers to see if they make sense and suggest next steps, which are specific to the problem and the objective. The example should cover this well.

B. Example

Problem: Company X is planning to develop a product for male baldness (to stop hair loss in males) and wants to understand the size of the opportunity.

Task: estimate the size of the male baldness market in the United States in terms of number of males currently buying these products.

I would pause here and attempt to estimate the size of this market before looking at the discussion.

C. Discussion

We will start with the rationale, followed by assumptions and calculations, conclusions and next steps:

1. Rationale:

We can pick three drivers for this estimation: US population, income and age:

• US population: straightforward, as the company is interested in the US market
• Income: important as anti-baldness products are non first-necessity products, so our hypothesis is that the majority of people in the poorest segment of the population cannot afford them
• Age: affects two variables that impact the size of the market: frequency of baldness and beauty consciousness. Our hypotheses are: baldness increases as people get older, but beauty consciousness decreases with age

2. Assumptions and calculations

Assumptions:

US population: The USA has approximately 300 million people, 50% female and 50% male

Income: The poorest segment represents 20% of the US population and they cannot afford beauty products in general (which includes anti-baldness products)

Age: Average life expectancy=80 years; Four age ranges (0-20, 21-40, 41-60, 61-80) assuming equal number of people in each

Calculations:

The logic behind the calculations follow your initial hypotheses:

Income: we can remove the 20% (poorer segment) from the US population= 300 million * 80%= 240 million

Age: out of the 240 million left, 120 million are males (50%). We can then distribute the males on the four age brackets and apply the percentages for incidence of baldness and willingness to buy the product (beauty consciousness) according to our initial hypotheses, as shown on the table below:

3. Conclusions and next steps

There are approximate 5 million males in the US buying anti-hair loss products. However, this is not enough to address our question (how attractive the opportunity is). The next steps are:

• Market size in revenue terms: we need the frequency of use (how many products per customer/year) x price.
• Market growth rate
• Market share: understand the share we are likely to capture, influenced by factors such as: competitive landscape: how many competitors, their products and unique selling points versus ours, etc

Final thoughts

In a case interview, you don't want to pick many drivers (otherwise your calculations etc will become a nightmare)....usually 2-3 are enough as long as you can explain the WHY part well. At the end of your estimation, your inteviewer may ask you a structure brainstorming question about ideas of other drivers which may influence the size of a market. In this case we can think of:

• Cultural factors: some cultures are more beauty conscious than others
• Geography: people in urban areas may be more beauty conscious than rural
• Occupation: people in office jobs may be more beauty conscious versus factory workers for example, although that may change with the remote working trends posed by COVID-19 :)

It is worth mentioning this is not the only way to do it; the important thing is you follow a logic and structured approach, are accurate in your calculations, interpret the results and put them in the context of the problem.

Hope that helps!

Let me know if you have any additional questions, and all the best with your case preping!

(edited)